Duty on one mode of transport (4)
I believe the answer is:
taxi
'mode of transport' is the definition.
'taxi' can be an answer for 'transport' (taxi is a kind of transport). I'm not certain of the 'mode of' bit.
'duty on one' is the wordplay.
'duty' becomes 'tax' (I've seen this before).
'on' means one lot of letters go next to another.
'one' becomes 'i' (Roman numeral).
'tax'+'i'='TAXI'
Can you help me to learn more?
(Other definitions for taxi that I've seen before include "Danny DeVito sitcom" , "(Of a plane) move slowly on the ground" , "Cab for hire" , "Hackney cab" , "use apron" .)