Duty imposed on one vehicle (4)
I believe the answer is:
taxi
'vehicle' is the definition.
(taxi is a kind of vehicle)
'duty imposed on one' is the wordplay.
'duty' becomes 'tax' (I've seen this before).
'imposed on' is a charade indicator (letters next to each other).
'one' becomes 'i' (Roman numeral**).
'tax'+'i'='TAXI'
(Other definitions for taxi that I've seen before include "Cab for hire" , "Hired vehicle" , "Car for hire" , "Minicab" , "type of vehicle" .)