It's in the transport business (3)

I believe the answer is:

bus

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'transport business' is the definition.
'bus' can be an answer for 'transport' (busing is a kind of transporting). I am not sure about the 'business' bit.

I don't understand the rest of the clue.

Can you help me to learn more?

(Other definitions for bus that I've seen before include "Road vehicle able to carry many passengers" , "eg Double-decker" , "Something caught" , "Mode of public transport" , "People carrier" .)

I've seen this clue in the Evening Standard.

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