Argon or radon (3)
I believe the answer is:
gas
This clue is a double definition.
'argon' is the first definition.
(thesaurus)
'radon' is the second definition.
(thesaurus)
(Other definitions for gas that I've seen before include "It may be natural" , "16 (US)" , "Something exciting" , "Chat (colloq.)" , "Neon, for example" .)