Jesting duke unknown to keep luxury car (8)
I believe the answer is:
drollery
'jesting' is the definition.
(both can mean joking)
'duke unknown to keep luxury car' is the wordplay.
'duke' becomes 'd' (abbreviation).
'unknown' becomes 'y' (algebra).
'to keep' indicates putting letters inside.
'luxury car' becomes 'roller' (short for Rolls Royce).
'd'+'y'='dy'
'dy' enclosing 'roller' is 'DROLLERY'.
(Other definitions for drollery that I've seen before include "Comical quality" , "display of humour" , "Jesting" , "comedy" , "Jest; buffoonery" .)