Cap removed from toy firearm (5)
I believe the answer is:
rifle
'firearm' is the definition.
(rifle is a kind of firearm)
'cap removed from toy' is the wordplay.
'cap removed from' means to remove the first letter (the top letter or 'cap' is removed).
'toy' becomes 'trifle' (both can mean to play with).
'trifle' with its initial letter taken off is 'RIFLE'.
(Other definitions for rifle that I've seen before include "Gun with a long barrel" , "Gun; thoroughly search" , "Flier (anag.)" , "Carbine" , "Shoulder firearm" .)