Jet fuel? (4)
I believe the answer is:
coal
I believe this clue is a double definition.
'jet' is the first definition.
(thesaurus)
'fuel?' is the second definition.
(coal is a kind of fuel)
(Other definitions for coal that I've seen before include "Carbonaceous rock used as fuel" , "Carbonized vegetable matter" , "Char" , "slack" , "Three domestic fuels" .)