Neon and xenon (5)
I believe the answer is:
gases
'neon and xenon' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for gases that I've seen before include "Vapours" , "Hydrogen and carbon monoxide, say" , "Neon, oxygen, etc" , "Air-like fluids" , "Eg, helium, nitrogen" .)