Leonard shot Mexican bandit once (7)
I believe the answer is:
ladrone
'mexican bandit once' is the definition.
(historical term for a bandit)
'leonard shot' is the wordplay.
'shot' indicates an anagram.
'leonard' with letters rearranged gives 'LADRONE'.
(Other definitions for ladrone that I've seen before include "Old pirate" , "robber" , "he'd nip there once" .)