Gas used in welding (9)
I believe the answer is:
acetylene
'gas used in welding' 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 "Flammable gas" , "powers industrial torch" , "Illuminating stuff also used for joining" , "Gas used chiefly in welding" , "cutting gas emission?" .)