A permit held by the runner? (7)
I believe the answer is:
'runner?' is the definition.
(runner is a kind of athlete)
'a permit held by the' is the wordplay.
'permit' becomes 'let' ('let' can be a synonym of 'permit').
'held by' is an insertion indicator.
'let' placed within 'the' is 'thlete'.
(Other definitions for athlete that I've seen before include "the hare that lost, say" , "Bolt perhaps" , "Track and field competitor" , "Strangely, the tale of one who is fit" , "Sport participant" .)