Olympian allowed to enter different heat (7)
I believe the answer is:
athlete
'olympian' is the definition.
(I've seen this before)
'allowed to enter different heat' is the wordplay.
'allowed to' becomes 'let' (to let something happen is to allow it).
'enter' indicates putting letters inside.
'different' indicates anagramming the letters.
'heat' anagrammed gives 'athe'.
'let' going into 'athe' is 'ATHLETE'.
(Other definitions for athlete that I've seen before include "gymnast, perhaps" , "Sportsman in track or field events" , "Track or field sportsperson" , "front-runner, possibly" , "Sprinter, say" .)