Government department seconds fashion-conscious youths (4)
I believe the answer is:
mods
'fashion-conscious youths' is the definition.
(members of the mod subculture)
'government department seconds' is the wordplay.
'government department' becomes 'mod' (Ministry of Defence).
'seconds' becomes 's'.
'mod'+'s'='MODS'
(Other definitions for mods that I've seen before include ". . . . and rockers, youth subcultures in the 1960s" , "1960s' youth subculture involving motor scooters" , "Highland festivals; Oxford exams" .)