Looter's assistant breaks into expensive car (6)
I believe the answer is:
raider
'looter's' is the definition.
(synonyms)
'assistant breaks into expensive car' is the wordplay.
'assistant' becomes 'aide' (aide is a kind of assistant).
'breaks into' means one lot of letters goes inside another (inserted letters break into the word).
'expensive car' becomes 'rr' (short for Rolls Royce).
'aide' placed within 'rr' is 'RAIDER'.
(Other definitions for raider that I've seen before include "One plundering" , "Maker of surprise attack" , "Pillager, plunderer" , "One who makes a sudden swift attack" , "Surprise attacker" .)