Type of engine (6)
I believe the answer is:
diesel
'type of engine' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for diesel that I've seen before include "Train perhaps" , "Derv" , "Rudolf __ , French-born German engine inventor, d.1913" , "Type of engine" , "See lid, oddly, of the fuel" .)