One of the inert gases (4)
I believe the answer is:
neon
'one of the inert gases' 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 "Small plant; colour" , "It may illuminate" , "Atomic number 10" , "Type of lamp" , "Colourless, odourless rare gas" .)