Ministry of Defence's scooter squad (4)
I believe the answer is:
mods
'scooter squad' is the definition.
(the mod subculture is famous for using scooters)
'ministry of defence's' is the wordplay.
'ministry' becomes 'mod' (Ministry of Defence).
'of defence's' becomes 's' (I can't justify this - if you can you should give a lot more credence to this answer).
'mod'+'s'='MODS'
(Other definitions for mods that I've seen before include ". . . . and rockers, youth subcultures in the 1960s" , "Highland festivals; Oxford exams" , "1960s' youth subculture involving motor scooters" .)