The Age Cryptic by DP #20,550

Just a quick post really because I'm flat out today. I got through this all right but got hung up ion the SW corner. There are still a few clues I've not been able to parse so if you can help me out, oh puzzling friends, please do! Here's my pic:

12A - Are they the legless tallymen?

I could guess from the definition that "tallymen" were ADDERS but wasn't sure what "are they the legless" was about. I thought maybe "legless" was akin to rungs on a ladder and we were to drop the "L"? Or is a reference to snakes?

3D - NY skyscraper is state inserted into status enhancement (6,8)

With 1A putting an E in the first place I guessed Empire State Building right off but only put in EMPIRE because I wasn't sure how the clue was working. Still not sure what going on to get me the answer.

15D - Reversion to unseat footballer (9)

Very confused as I thought "reversion" was an anagrind but it's not. Then thought it was a reversal signpost but it's not. "footballer" is such a vague term as there are four sport claiming in area or another to be "football" I didn't know which sport and couldn't ever single out a player. Nope if this clue was a fish then I'd Throw it Back.

I did find plenty to like in this puzzle like 9D (A SHOT IN THE DARK) and 16A (EYE-LINERS) and like the DH was good practice.

DA tomorrow, fingers crossed for a sharp mind. Wishing you the same.