Spin with monarch in posh car (6)
I believe the answer is:
roller
'posh car' is the definition.
(abbreviation for Rolls Royce)
'spin with monarch' is the wordplay.
'spin' becomes 'roll'.
'with' is a charade indicator (letters next to each other).
'monarch' becomes 'ER' (abbreviation for Elizabeth Regina, British Queen).
'roll'+'er'='ROLLER'
'in' acts as a link.
(Other definitions for roller that I've seen before include "Rotating cylinder" , "Painting tool" , "Cylinder for curling the hair" , "luxury car?" , "Long heavy wave approaching the shore" .)