Long jumper perhaps allowed to enter running heat (7)
I believe the answer is:
athlete
'long jumper' is the definition.
(I've seen this before)
'allowed to enter running heat' is the wordplay.
'allowed to' becomes 'let' (to let something happen is to allow it).
'enter' indicates putting letters inside.
'running' indicates anagramming the letters (the letters run around to different places).
'heat' with letters rearranged gives 'athe'.
'let' placed within 'athe' is 'ATHLETE'.
'perhaps' is the link.
(Other definitions for athlete that I've seen before include "Person who competes in sports" , "expected to be agile, of course" , "One good at sports" , "the hare that lost, say" , "Eg, a runner or gymnast" .)