Advertising sign (4)
I believe the answer is:
neon
'advertising sign' 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 "'Chemical element, often used in lights (4)'" , "None like bright city light" , "Colourless noble gas" , "Inert gas, symbol Ne" , "Element that gives a red glow in a vacuum tube" .)