I got to this sometime before Good Game and set my limit to the end of Midsomer Murders, which was the wankiest episode ever so I moved over to something I didn't watch but used as background noise. Solution in hand by 11pm, save for 10A where I had all cross letters but couldn't decide on which answer of two was the right one, and no will to persevere. Here's what I had:
1A - They plan to change thermodynamic property? (8)
We are looking for a "Thermodynamic property" and looking at "They plan to change" is fodder and anagrind giving us: ENTHALPY. Quick google to verify if, like me, you're not a physicist.
5A - Boofhead! Ice is about for wild mountain goats! (6)
We want a breed of mountain goats. "Boofhead" is a one word instruction to get B - the head of boof, "ice is around" the B to give us IBICES. I've never heard of them.
9A - Captain Rudolph fixed the game equipment (5,3)
"Game equipment" is our definition. "Captain" = C and "Rudolph" = HESS and "fixed" = Set if we think of fixing broken limbs, perhaps to give us CHESS SET.
10A - Former star whipped spear carriers (6)
I didn't know which of two possible words, EXTRAS or EXERTS, was the right answer. I think this is an unusual clue because I can't see how from the clue we get an accurate definition. "Former" = EX then "whipped spear" = SPARE and then "SPARE carriers" = EXTRAS. But really only "spare" but how does "carriers" work here? Is this not about couriers/messenger type carrier? I'm sure there's something off with that so please tell me how you solved this.
12A- People Harris followers include Hungarian composer (5)
"Hungarian composer" is our definition. The word "include" is a hidden word signpost and look closer to "peopLE HARris" to get LEHAR.
14A - In prison, emu cooked? That's authentic? (5-4)
"That's authentic" is our definition and "In prison, emu cooked" is fodder and anagrind to give us SIMON PURE. Yet another new expression for me.
15A - Behind the bar, dear, arranged wedding for group of workers (5,5)
"Group of workers" is our definition "behind the bar" = T and I'm not exactly sure how so that needs some work. "dear, arranged" is part anagram and fodder, "wedding" = UNION, to get this:
T(RADE) UNION.
16A - Left inside to be married Tuesday, in the joint (4)
I got this wrong in my grid, mistaking "joint" as WEED. Oops! "Left inside" = L to be placed "inside" "to be married" = WED for WELD a far more acceptable version of joining.
18A - Ring New York at ten for banded chalcedony (4)
"Banded chalcedony" is the definition. "ring" = O, "New York" = NY, "ten" = X for ONYX.
21A - No? Beth is a problem for feature of Japanese horticulture (3,6)
"Feature of Japanese horticulture" is the definition and "No? Beth is a problem," is fodder and anagrind for
24A - Fine fellow from ancient city is pressing (6)
"Is pressing" is the definition and "Fine fellow" = GENT and "From ancient city" somehow becomes UR so let me in on that secret, please. *Edit: Ah, thanks to @Patrickavenell who says "Ur was an old city in the Middle East - I think it was the first city to have 1 million occupants." So there we are. A nugget of information to be savoured for future puzzling exploits. I'm willing to bet this shows up loads in future puzzles, as per Murphy's Law.
27A - Specialist to freeze lure (6)
"Lure" is the definition. "Specialist" is an ENT as in ear, nose, throat doctor and "freeze" = ICE for ENTICE.
28A - The last impression before tea-break? Having no teeth (8)
"Having no teeth" is the definition and "thE last" = E, "impression" = DENT and "tea-break" is part fodder and anagrind leaving us with EDENTATE.
1D - Surpass when said to be forty in ancient Rome (5)
"Surpass" is the definition. This is a homophone clue so when we say "forty in ancient Rome" we say XL which is like EXCEL.
2, 19A, 22A, 13D, 6D, 25A - Crazy Dave Chumpling hated wee Honey-Beth Oxford waiting for Rocky Marciano (3,10,5,11,6,8)
This is likely the largest anagram I've ever encountered. Luckily I didn't have to solve this clue that way. I grew up not far from Rocky Marciano's hometown. He was a local legend and my Dad was a boxing fan so there was no escaping Rocky. I knew straight off he was THE UNDEFEATED WORLD HEAVYWEIGHT BOXING CHAMPION, but just in case, I did my little line drawing thing to show this (as no anagram algorithm can sort this) it really is:
3D - Debra sob fitfully when taken in (8)
"When taken in" is our definition and "Debra sob fitfully" which has suspicious grammar is fodder and anagrind - always be suspicious of bad grammar. We get ABSORBED.
4D - Sent planes as movement revealed cordial treatment (12)
"Cordial treatment" is the definition. "Sent planes as movement," is fodder and anagrind "revealing" PLEASANTNESS.
7D - Bruce Tarr to adjust engine part (11)
"Engine part" is the definition and yes, another anagram "Bruce Tarr to adjust" is fodder and anagrind giving us: CARBURETTOR.
8D - Need spuds cooked or hung up (9)
Another anagram? Geez...ok well "Hung up" is the definition and "Need spuds cooked," is fodder and anagrind giving us SUSPENDED.
11D- Irish member finished? I took off when destitute (12)
"Destitute" is the definition. This is a pain of a clue. I have read that if you have to use five or more tricks to make a solve it's a bad clue so here goes: "Irish" = I, "Member" = MP as a member of parliament, "finished" = OVER, "I" = I, "took off" = SGED for IMPOVERISHED. So much easier to just find cross letters and use a thesaurus. Only freaks like me go through the effort to break down a clue like this in its entirety. You're welcome.
15D - Our turtle chopped and used to catch fish? (5,4)
Have we developed a sixth sense for anagrams now? Are the red flags flying, the alarm bells ringing? Right, this is more like words tossed together to sort of be a phrase if you squint or are quite gullible. "To catch fish" is the definition, "Our turtle chopped," becomes TROUT LURE.
17D - Won final? Wow! Mean to change and be a refreshed lady (3,5)
"Refreshed lady" is the definition and "woW, final" = W, "Wow mean to change" is part fodder and anagrind to give us: NEW WOMAN.
20D - Captain can go back for some fine brandy (6)
"Fine brandy" is our definition and "Captain" = C and "can go back" means to reverse the previous bits to get C(OGNAC)
23D - Alighieri stake 500 before? (5)
"Alighieri," is the definition and "stake" = ANTE, "500 before" = D to be place before ANTE for DANTE.
26D - Top class rough-head in ascending militant group (1.1.1.)
"Top class" = A, "Rough headed," = R and "in ascending" = I but I think it's just In = I and the "ascending" part means to put those in reverse in the grid from bottom to top giving us IRA the militant group. I've not seen "in" = I before, is that a thing?
So a few gaps in my knowledge. I'll give today's DP a run later. I need tea and a cake, thanks very much.

