Gas in store signs (4)
I believe the answer is:
neon
'gas in store 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 "Gas - light" , "Gas used in fluorescent lamps" , "One of the six inert gasses" , "Gaseous element, in advertising signs, say" , "Type of lighting" .)