Northerner is in haulage or diesel (7)
I believe the answer is:
geordie
'northerner' is the definition.
(term for a person from Tyneside in NE England)
'in haulage or diesel' is the wordplay.
'in' says the answer is hidden in the clue.
'GEORDIE' can be found hidden inside 'haulage or diesel'.
'is' acts as a link.
(Other definitions for geordie that I've seen before include "Native of Tyneside" , "Person from Tyneside" , "Native of Newcastle" , "Newcastle-born?" , "Newcastle dialect" .)