One in vehicle is conceited (4)
I believe the answer is:
vain
'is conceited' is the definition.
(synonyms)
'one in vehicle' is the wordplay.
'one' becomes 'i' (Roman numeral).
'in' indicates putting letters inside.
'vehicle' becomes 'van' (type of vehicle).
'i' inserted into 'van' is 'VAIN'.
(Other definitions for vain that I've seen before include "Pointless; narcissistic" , "without success" , "Useless; conceited" , "Empty" , "Forever looking in the mirror" .)