Best to leave a reversing car (4)
I believe the answer is:
merc
'car' is the definition.
(short for Mercedes)
I cannot understand how the remainder of the clue works.
(Other definitions for merc that I've seen before include "one hired for foreign work" , "High-performance German car" , "German make of luxury car (colloq.)" , "Former cop" , "business-class car" .)