Big rig's fuel (6)
I believe the answer is:
diesel
'big rig's fuel' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for diesel that I've seen before include "Motor fuel" , "Fuel for heavy vehicles like lorries" , "See lid for the engine" , "Rudolf __ , French-born German engine inventor, d.1913" , "Train perhaps" .)