Firearm found in sack (5)
I believe the answer is:
rifle
I believe this clue is a double definition.
'firearm' is the first definition.
(I know that rifle is a type of firearm)
'sack' is the second definition.
(both can mean to steal)
'found in' acts as a link.
(Other definitions for rifle that I've seen before include "Firearm - plunder" , "One that has stock" , "Security provider with stock" , "Steal and carry off - firearm" , "Shoulder firearm" .)