Duty on first vehicle (4)
I believe the answer is:
taxi
'vehicle' is the definition.
(taxi is a kind of vehicle)
'duty on first' is the wordplay.
'duty' becomes 'tax' (I've seen this before).
'on' is a charade indicator (letters next to each other).
'first' becomes 'i' (I in Roman numerals eg Elizabeth I**).
'tax'+'i'='TAXI'
(Other definitions for taxi that I've seen before include "Hired vehicle" , "(Of a plane) move slowly on the ground" , "run along the ground" , "Transport for hire" , "Prepare to take off" .)