Horseman arrests amateur looter (6)
I believe the answer is:
raider
'looter' is the definition.
('raider' can be a synonym of 'looter')
'horseman arrests amateur' is the wordplay.
'horseman' becomes 'rider' (I've seen this before).
'arrests' is an insertion indicator.
'amateur' becomes 'a'.
'rider' going around 'a' is 'RAIDER'.
(Other definitions for raider that I've seen before include "One plundering" , "person taking food from larder" , "One who makes surprise attack, perhaps to steal" , "Viking" , "Maker of surprise attack" .)