Jeans brand since 1978 (6)
I believe the answer is:
diesel
'jeans brand since 1978' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for diesel that I've seen before include "Engine fuel" , "Train perhaps" , "Motor fuel" , "See lid, oddly, of the fuel" , "Lorry fuel" .)