Gas in some lights (4)
I believe the answer is:
neon
'gas in some lights' 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 "Gas - light" , "Inert gas, symbol Ne" , "Colourless, odourless rare gas" , "Type of lamp" , "Gas used in fluorescent lighting" .)