Runner allowed to take part in different heat (7)
I believe the answer is:
athlete
'runner' is the definition.
(I've seen this before)
'allowed to take part in different heat' is the wordplay.
'allowed' becomes 'let' (I've seen this before).
'to take part in' is an insertion indicator.
'different' is an anagram indicator.
'heat' with letters rearranged gives 'athe'.
'let' placed into 'athe' is 'ATHLETE'.
(Other definitions for athlete that I've seen before include "Eg long jumper" , "Track and field competitor" , "Runner, jumper, thrower" , "Track/field sportsman" , "Holmes, perhaps" .)