Welding gas (9)
I believe the answer is:
acetylene
'welding gas' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for acetylene that I've seen before include "Gas used chiefly in welding" , "Flammable gas" , "Illuminating stuff also used for joining" , "powers industrial torch" , "cutting gas emission?" .)