Sprinter, maybe, allowed to enter rearranged heat (7)
I believe the answer is:
athlete
'sprinter maybe' is the definition.
'allowed to enter rearranged heat' is the wordplay.
'allowed' becomes 'let' (I've seen this before).
'to enter' is an insertion indicator.
'rearranged' indicates anagramming the letters.
'heat' anagrammed gives 'athe'.
'let' put into 'athe' is 'ATHLETE'.
(Other definitions for athlete that I've seen before include "gymnast, perhaps" , "Track eventer" , "Person trained to compete in sports" , "One trains in sport" , "Track/field sportsman" .)