Mine gas (8)
I believe the answer is:
firedamp
'mine gas' is the definition.
(I know that mine gas can be written as firedamp)
This is all the clue.
(Other definitions for firedamp that I've seen before include "Explosive gas in mines" , "Methane found in coal mines" , "Something potentially dangerous in mines" , "Combustible mixture" , "Methane in coalmines" .)