Underling working under car (6)
I believe the answer is:
minion
'underling' is the definition.
(thesaurus)
'working under car' is the wordplay.
'working' becomes 'on'.
'under' means one lot of letters go next to another.
'car' becomes 'mini' (Mini Cooper**).
'on' put after 'mini' is 'MINION'.
(Other definitions for minion that I've seen before include "Flunky" , "Fawning servant" , "Servile dependant" , "Lowly worker" , "Servile underling" .)