Broadway brightener (4)
I believe the answer is:
neon
'broadway brightener' 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 (anag.)" , "It may illuminate" , "Gas used in fluorescent lighting" , "Could it brighten proceedings" , "Gaseous element, in advertising sign say" .)