Not opening, toy gun (5)
I believe the answer is:
rifle
'gun' is the definition.
(I know that rifle is a type of long gun)
'not opening toy' is the wordplay.
'not opening' means to remove the first letter (the opening letter is removed).
'toy' becomes 'trifle' (both can mean to play with).
'trifle' with its first letter taken off is 'RIFLE'.
(Other definitions for rifle that I've seen before include "Gun with long_barrel" , "Long-barrelled gun" , "Search with intent to steal" , "lock, stock and barrel" , "Search and rob for army weapon" .)