Smooth-bore weapon (7)
I believe the answer is:
shotgun
'smooth-bore weapon' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for shotgun that I've seen before include "arm" , "No thugs (anag) - weapon" , "Firearm used at short range - kind of wedding" , "Noughts (anag) - type of wedding" , "Type of wedding - weapon" .)