Although this has been posted, no correct answer has been given so thought I'd try to post...
The clue is simply, Athletic Aces and the number of letters are 4 S?A?.
The S is from Substitute (Reserve player) and the A from Arete (Mountain Ridge) so the answer cannot be Cram, as someone else suggested.
Thank You for all your efforts but I still don't think we have found the right answer. The answer will be the Surname of an Athletic's star, ie I have Gunnell for another clue (17a).
Some more head scratching!!!
Sorry Redman, think I confused you. I meant I had worked out that Gunnell was the answer. You don't actually get any clue for the Athletic Aces, you just have to work them out around the other clues you have, hence, I am left with the S?A? for 1d.
Thanks all the same.
Do you mean that the 'Theme' of the Crossword/puzzle is Athletic Aces ?
If it is, that means that the answer does not have to be plural, in which case, auntflo has given you a couple of names to consider.