Conceited, one sitting in vehicle (4)
I believe the answer is:
vain
'conceited' is the definition.
('vain' can be a synonym of 'conceited')
'one sitting in vehicle' is the wordplay.
'one' becomes 'i' (Roman numeral).
'sitting in' indicates putting letters inside.
'vehicle' becomes 'van' (van is a kind of vehicle).
'i' going into 'van' is 'VAIN'.
(Other definitions for vain that I've seen before include "Producing no result" , "being a big-head" , "Stuck-up? It's hopeless" , "Worthless" , "without success" .)