Frivolous duke towards the rear (4)
I believe the answer is:
daft
'frivolous' is the definition.
(both can mean silly)
'duke towards the rear' is the wordplay.
'duke' becomes 'd' (abbreviation).
'towards the rear' becomes 'aft' (I've seen this before).
'd'+'aft'='DAFT'
(Other definitions for daft that I've seen before include "Barmy, crackers" , "'---- as a brush': dotty" , "Potty" , "Stupid" , "Half-witted" .)