A gas burner! (4)
I believe the answer is:
coal
This might be a double definition.
'a gas' is the first definition.
(thesaurus)
'burner' is the second definition.
Although both the answer and definition are singular nouns, I cannot see how one could define the other.
Can you help me to learn more?
(Other definitions for coal that I've seen before include "Fossil fuel" , "Char" , "Carbon fuel" , "Carbonaceous rock used as fuel" , "Mined fuel" .)