Marsh gas (7)
I believe the answer is:
methane
'marsh gas' is the definition.
(I know that marsh gas can be written as methane)
This is the entire clue.
(Other definitions for methane that I've seen before include "contributor to global warming" , "Marsh-gas" , "Marsh gas - the name (anag)" , "Main constituent of natural gas" , "compound" .)