Exhaust on one vehicle (4)
I believe the answer is:
taxi
'vehicle' is the definition.
(taxi is a kind of vehicle)
'exhaust on one' is the wordplay.
'exhaust' becomes 'tax' (tax can mean to exhaust or demand too much from).
'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 "Hired vehicle" , "(Of a plane) move slowly on the ground" , "Hackney cab" , "Motor cab" , "use apron" .)