Inert element (5)
I believe the answer is:
argon
'inert element' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for argon that I've seen before include "An insert gas to groan about" , "something inert and insubstantial" , "Gaseous element" , "Noble gas used in light bulbs" , "Inert gas used in light bulbs" .)