Crossword Genius

Its driver usually has the knowledge (3)

Ross

I believe the answer is:

cab

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'knowledge' is the definition.
Although both the answer and definition are singular nouns, I don't understand how they can define each other.

I cannot understand how the remainder of the clue works.

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(Other definitions for cab that I've seen before include "An American taxi" , "Driving compartment" , "It has the authority to confiscate the proceeds of crime, in short" , "info for citizens provided here" , "Transport" .)

I've seen this clue in the Evening Standard.
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