Runner allowed to take part in a different heat (7)
I believe the answer is:
athlete
'runner' is the definition.
(I've seen this before)
'allowed to take part in a different heat' is the wordplay.
'allowed to' becomes 'let' (to let something happen is to allow it).
'take part in' is an insertion indicator.
'a different' is an anagram indicator (letters placed in a different order).
'heat' anagrammed gives 'athe'.
'let' put within 'athe' is 'ATHLETE'.
(Other definitions for athlete that I've seen before include "eg Dean Macey" , "Eg long jumper" , "Person trained to compete in sports" , "Track eventer" , "Person who competes in sports" .)