Gas used in lighting (4)
I believe the answer is:
neon
'gas used in lighting' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for neon that I've seen before include "One of the six inert gases" , "It may illuminate" , "Gaseous element, in advertising signs, say" , "Noble gas used in illuminated signs" , "Type of lighting" .)