Overload one’s hire car (4)

I believe the answer is:

taxi

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'hire car' is the definition.
(I've seen this before)

'overload one's' is the wordplay.
'overload' becomes 'tax' (I am not sure about this - if you are sure you should believe this answer much more).
'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 "Transport for hire" , "Public service vehicle" , "Vehicle with a meter" , "I hail you!" , "Motor cab" .)

I've seen this clue in The Sunday Times.

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