Because round leg not quite right, one's attached to arm (8)
I believe the answer is:
silencer
'one's attached to arm' is the definition.
(a silencer might be attached to a firearm)
I cannot understand how the rest of the clue works.
(Other definitions for silencer that I've seen before include "part of car" , "Exhaust quietener" , "Quieter" , "Device reducing a gun's noise" , "It muffles the sound of a gun" .)