Sportsman allowed to enter different heat (7)
I believe the answer is:
athlete
'sportsman' is the definition.
(thesaurus)
'allowed to enter different heat' is the wordplay.
'allowed' becomes 'let' (I've seen this before).
'to enter' is an insertion indicator.
'different' indicates anagramming the letters.
'heat' is an anagram of 'athe'.
'let' going inside 'athe' is 'ATHLETE'.
(Other definitions for athlete that I've seen before include "Runner, jumper, thrower" , "Olympic participant" , "Trained sportsperson" , "Eg, a runner or gymnast" , "Eg long jumper" .)