Bright sign (4)
I believe the answer is:
neon
'bright sign' 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 "Could it brighten proceedings" , "Element that gives a red glow in a vacuum tube" , "Type of lamp" , "Noble gas used in illuminated signs" , "Colourless, odourless rare gas" .)