Sound criterion for a weapon (6)
I believe the answer is:
cannon
'for a weapon' is the definition.
(cannon is a kind of weapon)
'sound criterion' is the wordplay.
'sound' indicates a 'sounds like' (homophone) clue.
'criterion' becomes 'canon' (I have seen 'accepted criterion' mean 'canon' so perhaps 'criterion' could also mean 'canon').
'canon' is a homophone of 'CANNON'.
(Other definitions for cannon that I've seen before include "mortar" , "Billiard shot or large gun" , "Snooker shot" , "Item of heavy artillery" , "Large gun that fires heavy shells" .)