Gas used in fluorescent lights (4)
I believe the answer is:
neon
'gas used in fluorescent lights' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for neon that I've seen before include "Form of lighting" , "Colourless gas, used in lighting" , "It may illuminate" , "Gas used in fluorescent lamps" , "Element that gives a red glow in a vacuum tube" .)