Gas used in signs (4)
I believe the answer is:
neon
'gas used in signs' 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 "Could it brighten proceedings" , "illuminating agent" , "Sign gas" , "One of the six inert gasses" , "None (anag.)" .)