Sportsmen allowed to be in staggered heats (8)
I believe the answer is:
athletes
'sportsmen' is the definition.
(I've seen this before)
'allowed to be in staggered heats' is the wordplay.
'allowed to' becomes 'let' (to let something happen is to allow it).
'be in' is an insertion indicator.
'staggered' indicates anagramming the letters.
'heats' is an anagram of 'athes'.
'let' placed within 'athes' is 'ATHLETES'.
(Other definitions for athletes that I've seen before include "Derval O'Rourke and Usain Bolt, say" , "Oddly, the least of it is they're very fit" , "Fungal infection" , "Eels that are good at sports" , "sportspeople?" .)