Arm mangled in brace (7)
I believe the answer is:
carbine
'arm' is the definition.
(carbine is a kind of arm)
'mangled in brace' is the wordplay.
'mangled' indicates an anagram.
'in'+'brace'='inbrace'
'inbrace' with letters rearranged gives 'CARBINE'.
(Other definitions for carbine that I've seen before include "A short light automatic rifle" , "Light automatic rifle -- nice bra (anag)" .)