This gun is almost an insignificant thing (5)
I believe the answer is:
rifle
'this gun' is the definition.
(rifle is a kind of gun)
'almost an insignificant thing' is the wordplay.
'almost' means to remove the first letter.
'an insignificant thing' becomes 'trifle' (I've seen this in another clue).
'trifle' with its initial letter taken away is 'RIFLE'.
'is' acts as a link.
(Other definitions for rifle that I've seen before include "Strip - firearm" , "Carbine" , "Gun; sack" , "An arm" , "Gun - ransack" .)