Anthracite or lignite (4)
I believe the answer is:
coal
I believe this clue is a double definition.
'anthracite' is the first definition.
(I've seen this in another clue)
'lignite' is the second definition.
(I've seen this in another clue)
(Other definitions for coal that I've seen before include "Carbon fuel" , "Three domestic fuels" , "slack" , "This consists mainly of carbon" , "Black mineral used for burning" .)