Buses and trains (7)
I believe the answer is:
transit
'buses and trains' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for transit that I've seen before include "The act of carrying from one place to another" , "Change" , "Conveyance of goods or people - tin star (anag)" , "Passage of heavenly body" , "Pass across/through" .)