Heat black fossil fuel (4)
I believe the answer is:
boil
'heat' is the definition.
(thesaurus)
'black fossil fuel' is the wordplay.
'black' becomes 'b' (as in pencil classifications).
'fossil fuel' becomes 'oil' (I've seen this before).
'b'+'oil'='BOIL'
(Other definitions for boil that I've seen before include "let off steam" , "become inflamed" , "Eruption of the skin" , "7 in water" , "Cook in hot water" .)