Runner allowed to enter the re-run heat (7)
I believe the answer is:
athlete
'runner' is the definition.
(I've seen this in another clue)
'allowed to enter the re-run heat' is the wordplay.
'allowed to' becomes 'let' (to let something happen is to allow it).
'enter' is an insertion indicator.
'the re-run heat' becomes 'athe' (I can't justify this - if you can you should give a lot more credence to this answer).
'let' placed into 'athe' is 'ATHLETE'.
Can you help me to learn more?
(Other definitions for athlete that I've seen before include "Person who competes in sports" , "Eg, a runner or gymnast" , "the hare that lost, say" , "gymnast, maybe" , "gymnast, perhaps" .)