Duty paid on current vehicle (4)
I believe the answer is:
taxi
'vehicle' is the definition.
(taxi is a kind of vehicle)
'duty paid on current' is the wordplay.
'duty' becomes 'tax' (I've seen this before).
'paid on current' becomes 'i' (I am not sure about this - if you are sure you should believe this answer much more).
'tax'+'i'='TAXI'
Can you help me to learn more?
(Other definitions for taxi that I've seen before include "Motor vehicle by rank?" , "Move along a runway" , "(Plane) roll over tarmac" , "Cab for hire" , "Car for hire" .)