Crossword Genius

Business picking up? (4)

Ross

I believe the answer is:

taxi

I'm a little stuck... Click here to teach me more about this clue!

This could be a double definition.

'business picking' is the first definition.
Although both the answer and definition are singular nouns, I cannot understand how they can define each other.

'up?' is the second definition.
Although both the answer and definition are verbs in their base form, I don't understand how they can define each other.

Can you help me to learn more?

(Other definitions for taxi that I've seen before include "US currency" , "Part of fleet" , "Move along a runway" , "Public service vehicle" , "Rank occupant" .)

I've seen this clue in The Sun.
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