Runner allowed to enter rearranged heat (7)
I believe the answer is:
athlete
'runner' is the definition.
(I've seen this before)
'allowed to enter rearranged heat' is the wordplay.
'allowed to' becomes 'let' (to let something happen is to allow it).
'enter' indicates putting letters inside.
'rearranged' indicates an anagram.
'heat' with letters rearranged gives 'athe'.
'let' put within 'athe' is 'ATHLETE'.
(Other definitions for athlete that I've seen before include "E.g. runner or jumper" , "One competes in sports" , "Player" , "gymnast, perhaps" , "fine physical specimen" .)