Gas in glowing signs (4)
I believe the answer is:
neon
'gas in glowing 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 "Brightly coloured advertising sign" , "Gaseous element, in advertising sign say" , "Colourless gas, used in lighting" , "Element 10, used in lights" , "None like bright city light" .)