Sportsmen allowed in new heats (8)
I believe the answer is:
athletes
'sportsmen' is the definition.
(I've seen this before)
'allowed in new heats' is the wordplay.
'allowed' becomes 'let' (to let is to allow or permit).
'in' means one lot of letters goes inside another.
'new' indicates an anagram.
'heats' anagrammed gives 'athes'.
'let' placed inside 'athes' is 'ATHLETES'.
(Other definitions for athletes that I've seen before include "Sports contestants" , "Derval O'Rourke and Usain Bolt, say" , "Fungal infection" , "Al, the set is up for the physically skilled" , "Sportsmen and women" .)