Adult allowed to wear most of these jumpers? (8)
I believe the answer is:
athletes
'jumpers?' is the definition.
(jumper is a kind of athlete)
'adult allowed to wear most of these' is the wordplay.
'adult' becomes 'a' (abbreviation).
'allowed to' becomes 'let' (to let something happen is to allow it).
'wear' means one lot of letters goes inside another (to wear something is to be covered up by it).
'most of' means to remove the last letter.
'these' with its last letter taken off is 'thes'.
'let' inserted within 'thes' is 'thletes'.
'a'+'thletes'='ATHLETES'
(Other definitions for athletes that I've seen before include "sportspeople?" , "Steal the strangely fit people" , "Eels that are good at sports" , "Oddely, the least of it is they're fit" , "Derval O'Rourke and Usain Bolt, say" .)