It's a lighter gas (6)
I believe the answer is:
butane
'it's a lighter gas' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for butane that I've seen before include "Bottled gas used as a fuel" , "Lighter fuel" , "Flammable gas in petroleum" , "Inflammable gas produced from petrol" .)