Form of transport levy that's imposed on one (4)
I believe the answer is:
taxi
'form of transport' is the definition.
(I know that taxi is a type of transport)
'levy that's imposed on one' is the wordplay.
'levy' becomes 'tax' (tax is a kind of levy).
'that's imposed 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 "I hail you!" , "At airport, guide" , "Part of fleet" , "type of vehicle" , "move slowly along the ground" .)