Krypton or xenon
I believe the answer is:
gas
This clue is a double definition.
'krypton' is the first definition.
(thesaurus)
'xenon' is the second definition.
(thesaurus)
(Other definitions for gas that I've seen before include "Hydrogen, for example" , "Oxygen maybe" , "Nitrogen, say" , "rabbit" , "Something exciting" .)