Fuel finishes mid-pipeline (6)
I believe the answer is:
diesel
'fuel' is the definition.
(diesel fuel is a kind of fuel)
'finishes mid-pipeline' is the wordplay.
'finishes' becomes 'dies'.
'mid' means to look at the middle letters (mid- can mean the middle of something).
The centre of 'pipeline' is 'el'.
'dies'+'el'='DIESEL'
(Other definitions for diesel that I've seen before include "Liquid fuel" , "Engine fuel" , "See lid, oddly, of the fuel" , "See lid for the engine" , "Train perhaps" .)