Attachment to arm when bent, reclines (8)
I believe the answer is:
silencer
'attachment to arm' is the definition.
(attachment to a weapon)
'bent reclines' is the wordplay.
'bent' is an anagram indicator.
'reclines' anagrammed gives 'SILENCER'.
'when' is the link.
(Other definitions for silencer that I've seen before include "Device designed to deaden sound" , "Report suppressor" , "Exhaust quietener" , "part of car" , "Reclines (anag.)" .)