Attending events with becoming old hat (6)

I haven't cracked this one yet — but the brilliant team at Fifteensquared have:

With a considerable amount of help it is a double definition. We had to reveal the answer for 15d as we were stuck on 15d and 15a. Once we had the initial letter we guessed it was DATING (becoming old hat) but we were not confident that ‘attending events with’ is really the same as ‘going out with’ in a romantic relationship