Hired vehicle: duty to take one (4)
I believe the answer is:
taxi
'hired vehicle' is the definition.
(I've seen this before)
'duty to take one' is the wordplay.
'duty' becomes 'tax' (I've seen this before).
'to take' means one lot of letters go next to another.
'one' becomes 'i' (Roman numeral**).
'tax'+'i'='TAXI'
(Other definitions for taxi that I've seen before include "Prepare to fly" , "Cab for hire" , "US currency" , "run along the ground" , "people carrier?" .)