Sprinter, perhaps, allowed in running heat (7)
I believe the answer is:
athlete
'sprinter perhaps' is the definition.
(sprinter is a kind of athlete)
'allowed in running heat' is the wordplay.
'allowed' becomes 'let' (I've seen this before).
'in' means one lot of letters goes inside another.
'running' is an anagram indicator.
'heat' anagrammed gives 'athe'.
'let' placed inside 'athe' is 'ATHLETE'.
(Other definitions for athlete that I've seen before include "One who engages in physical sports" , "Track eventer" , "Sportsman in track or field events" , "One competes in sports" , "Contestant" .)