It's in the business of transportation (3)
I believe the answer is:
bus
'transportation' is the definition.
The definition and answer can be both man-made objects as well as being singular nouns.
Perhaps they are linked in a way I don't understand?
I can't explain the remainder of the clue.
Can you help me to learn more?
(Other definitions for bus that I've seen before include "Public service vehicle" , "Means of public transport" , "Mode of public transport" , "Of projectiles; (go) mad (slang)" , "Road vehicle able to carry many passengers" .)