Numberless aches following long jump when Olympics are held (4,5)
I believe the answer is:
leap years
'when olympics are held' is the definition.
(Olympic tournaments are held in leap years)
'numberless aches following long jump' is the wordplay.
'numberless aches' becomes 'years' (I can't explain this - if you can you should give a lot more credence to this answer).
'following' means one lot of letters go next to another.
'long jump' becomes 'leap' (I've seen this in another clue).
'years' after 'leap' is 'LEAP-YEARS'.
(Another definition for leap years that I've seen is " 366-day periods".)