Damage is a broken arm (3)
I believe the answer is:
mar
'damage' is the definition.
(to mar is to spoil or damage)
'broken arm' is the wordplay.
'broken' indicates anagramming the letters.
'arm' anagrammed gives 'MAR'.
'is a' acts as a link.
(Other definitions for mar that I've seen before include "Spoil something" , "Ram into and spoil" , "Make imperfect" , "Cause harm" , "Blemish" .)