A car spares outlet may have one over a barrel! (8)
I believe the answer is:
silencer
This clue is a double definition.
'a car spares outlet may have' is the first definition.
(device used to reduce noise from a car engine)
'one over a barrel' is the second definition.
(a silencer might cover the barrel of a gun)
(Other definitions for silencer that I've seen before include "Device to muffle sound of gun" , "Car exhaust device" , "I keep report quiet" , "Fitment for exhaust or gun barrel" , "Device fitted to exhausts" .)