Runner let into swimming heat (7)
I believe the answer is:
athlete
'runner' is the definition.
(I've seen this before)
'let into swimming heat' is the wordplay.
'into' means one lot of letters goes inside another.
'swimming' indicates anagramming the letters.
'heat' anagrammed gives 'athe'.
'let' going within 'athe' is 'ATHLETE'.
(Other definitions for athlete that I've seen before include "Sporty type" , "One who's good at sport" , "expected to be agile, of course" , "front-runner, possibly" , "One trains in sport" .)