Militia crazy to jettison leader (4)
I believe the answer is:
army
'militia' is the definition.
(I've seen this in another clue)
'crazy to jettison leader' is the wordplay.
'crazy' becomes 'barmy' (I've seen this before).
'to jettison leader' means to remove the first letter (the letter which leads the word is removed).
'barmy' with its first letter taken off is 'ARMY'.
(Other definitions for army that I've seen before include "service" , "Military land force" , "See 21" , "Vast number" , "Large host" .)