Arm in error, if leg! (5)
I believe the answer is:
rifle
'arm' is the definition.
(rifle is a kind of arm)
'in error if leg' is the wordplay.
'in' says the answer is hidden in the clue.
'RIFLE' is hidden within 'error if leg'.
(Other definitions for rifle that I've seen before include "Steal and carry off - firearm" , "Shoulder firearm" , "Search with intent to steal" , "Ransack; a weapon" , "Weapon - sack" .)