Vehicle could be an imposition on one (4)
I believe the answer is:
taxi
'vehicle' is the definition.
(taxi is a kind of vehicle)
'an imposition on one' is the wordplay.
'an imposition on' becomes 'tax' (I can't explain this - if you can you should give a lot more credence to this answer).
'one' becomes 'i' (Roman numeral).
'tax'+'i'='TAXI'
'could be' is the link.
Can you help me to learn more?
(Other definitions for taxi that I've seen before include "move slowly along the ground" , "Public service vehicle" , "US currency" , "Car for hire" , "run along the ground" .)