Lorry driver (7)
(Other definitions for haulier that I've seen before include "Trucker" , "Commercial transporter" , "Trucking company" , "One transports goods in a vehicle" , "He could transport" .)
'lorry driver' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for trucker that I've seen before include "I convey" , "(US) lorry driver" , "One who has dealings with" , "Long-distance lorry driver" , "Professional driver" .)