Gas in signs (4)
I believe the answer is:
neon
'gas in signs' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for neon that I've seen before include "Noble gas used in illuminated signs" , "Element 10, used in lights" , "Gas used in fluorescent lights" , "illuminating agent" , "One of the six inert gasses" .)