Crossword Genius

In which business is picking up? (3)

Ross

I believe the answer is:

cab

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'in which business is picking up?' is the definition.
Although both the answer and definition are singular nouns, I don't understand how they can define each other.

This is all the clue.

Can you help me to learn more?

(Other definitions for cab that I've seen before include "Driving compartment" , "'The authority collects money made from crime, initially (3)'" , "An American taxi" , "info for citizens provided here" , "A taxi in New York" .)

I've seen this clue in The Sun.
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