Lighting gas (4)
I believe the answer is:
neon
'lighting gas' 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 "None like bright city light" , "Gaseous element, in advertising sign say" , "Brightly coloured advertising sign" , "Gaseous element, in advertising signs, say" , "Noble gas used in illuminated signs" .)