Runner possibly allowed to enter reorganised heat (7)
I believe the answer is:
athlete
'runner possibly' is the definition.
(runner is a kind of athlete)
'allowed to enter reorganised heat' is the wordplay.
'allowed' becomes 'let' (I've seen this before).
'to enter' indicates putting letters inside.
'reorganised' indicates anagramming the letters.
'heat' anagrammed gives 'athe'.
'let' going within 'athe' is 'ATHLETE'.
(Other definitions for athlete that I've seen before include "One may run" , "Track/field sportsman" , "One trains in sport" , "Track or field sportsperson" , "Sport participant" .)