Broken arm hurt (3)
I believe the answer is:
mar
'hurt' is the definition.
(both can mean to harm)
'broken arm' is the wordplay.
'broken' is an anagram indicator.
'arm' with letters rearranged gives 'MAR'.
(Other definitions for mar that I've seen before include "Spoil or deface" , "Detract from" , "Spanish main?" , "hurt" , "'Damage, impair (3)'" .)