Gas light (4)
I believe the answer is:
neon
'gas light' 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 "Noble gas used in illuminated signs" , "One of the six inert gasses" , "None like bright city light" , "Gas used in fluorescent lighting" , "Element that gives a red glow in a vacuum tube" .)