Trader knocking 50% off car (4)
I believe the answer is:
merc
'car' is the definition.
(short for Mercedes)
'trader knocking 50% off' is the wordplay.
'trader' becomes 'merchant' (a merchant is a trader or seller of goods).
'knocking 50% off' suggests halving the letters.
'merchant' cut in half is 'MERC'.
(Other definitions for merc that I've seen before include "German car (abbr)" , "one hired for foreign work" , "business-class car" , "Former cop" , "High-performance German car" .)