Glowing gas (4)
I believe the answer is:
neon
'glowing gas' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for neon that I've seen before include "None (anag.)" , "One of the six inert gasses" , "Gas used in fluorescent lighting" , "Brightly coloured advertising sign" , "Noble gas used in illuminated signs" .)