Injure breaking an arm (3)
I believe the answer is:
mar
'injure' is the definition.
(I know that mar is a kind of injure)
'breaking an arm' is the wordplay.
'breaking' is an anagram indicator.
'arm' anagrammed gives 'MAR'.
(Other definitions for mar that I've seen before include "when spring growth starts?" , "Spoil stuff" , "wreck" , "Spoil something" , "Ram into and spoil" .)