Impose a levy on one cab (4)
I believe the answer is:
taxi
'one cab' is the definition.
(synonyms)
'impose a levy on' is the wordplay.
'impose' becomes 'i' (this could be a standard abbreviation which I don't know about).
'a levy' becomes 'tax' (tax is a kind of levy).
'on' is a charade indicator (letters next to each other).
'i' after 'tax' is 'TAXI'.
Can you help me to learn more?
(Other definitions for taxi that I've seen before include "Part of fleet" , "I hail you!" , "Hackney cab" , "Prepare to fly" , "US currency" .)