One business that's picking up (4)
I believe the answer is:
taxi
'up' is the definition.
Although both the answer and definition are verbs in their base form, I cannot understand how they can define each other.
'one business that's picking' is the wordplay.
I cannot quite see how this works, but
'one' could be 'i' (Roman numeral) and 'i' is present in the answer.
The remaining letters 'tax' is a valid word which might be clued in a way I don't see.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for taxi that I've seen before include "Transport for hire" , "Rank occupant" , "I hail you!" , "move on ground before take-off" , "At airport, guide" .)