Vehicle one can never quite back up? (3)

I believe the answer is:

cab

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'vehicle' is the definition.
(cab is a kind of vehicle)

I cannot understand how the remainder of the clue works.

(Other definitions for cab that I've seen before include "It has the authority to confiscate the proceeds of crime, in short" , "A taxi in America" , "Hackney" , "An American taxi" , "A taxi in New York" .)

I've seen this clue in the Evening Standard.

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