Really bright (4)
I believe the answer is:
neon
'really bright' 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 "One of the six inert gases" , "Gas used in fluorescent lighting" , "Small plant; colour" , "Gas used in fluorescent lamps" , "Gas used in fluorescent lights" .)