Name the mixture producing marsh-gas (7)
I believe the answer is:
methane
'marsh-gas' is the definition.
(I've seen this before)
'name the mixture' is the wordplay.
'mixture' indicates anagramming the letters.
'name'+'the'='namethe'
'namethe' anagrammed gives 'METHANE'.
'producing' is the link.
(Other definitions for methane that I've seen before include "contributor to global warming" , "compound" , "Greenhouse gas produced by cows" , "Firedamp" , "Marsh gas - the name (anag)" .)