Duty on one vehicle (4)
I believe the answer is:
taxi
'vehicle' is the definition.
(taxi is a kind of vehicle)
'duty on one' is the wordplay.
'duty' becomes 'tax' (I've seen this before).
'on' says to put letters next to each other.
'one' becomes 'i' (Roman numeral**).
'tax'+'i'='TAXI'
(Other definitions for taxi that I've seen before include "Handgun" , "Minicab" , "Rank occupant" , "At airport, guide" , "Motor cab" .)