Public transport vehicle (3)
'public transport vehicle' is the definition.
(buses are a type of public transport)
This is the entire clue.
(Other definitions for bus that I've seen before include "Use transport" , "Motor-coach" , "Road vehicle able to carry many passengers" , "Public transport vehicle" , "Coach" .)
(Other definitions for omnibus that I've seen before include "Old vehicle - volume of works" , "Anthology" , "public service" , "Collection of books reprinted in one volume" , "Large vehicle - collected edition" .)
(Another definition for trolleybus that I've seen is " Electrically driven public service vehicle".)