Mountie catching a thief (6)
I believe the answer is:
raider
'thief' is the definition.
(I know that raider is a type of thief)
'mountie catching a' is the wordplay.
'mountie' becomes 'rider' (member of the Royal Canadian Mounted Police).
'catching' means one lot of letters goes inside another.
'rider' going around 'a' is 'RAIDER'.
(Other definitions for raider that I've seen before include "One attacks" , "Surprise attacker" , "Invader" , "person who's involved in sacking?" , "Maker of surprise attack" .)