In which business is picking up? (3)
I believe the answer is:
cab
'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" .)