City light (4)
I believe the answer is:
neon
'city light' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for neon that I've seen before include "Could it brighten proceedings" , "Sign gas" , "Colourless gas, used in lighting" , "Colourless, odourless rare gas" , "One of the six inert gasses" .)