Transport business (7)
(Other definitions for traffic that I've seen before include "Movement of vehicles" , "Deal in, trade" , "Moving vehicles; trade" , "What uses road" , "Commerce; transport" .)
'transport business' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for haulage that I've seen before include "Commercial transportation of goods" , "Road transport" , "Transport business" , "Transportation of heavy goods" , "Commercial transport of goods" .)