Helium or hydrogen (3)
I believe the answer is:
gas
This clue is a double definition.
'helium' is the first definition.
(I've seen this in another clue)
'hydrogen' is the second definition.
(hydrogen is a gas)
(Other definitions for gas that I've seen before include "Idle chat" , "could be embarrassing for American" , "Substance like air" , "Hydrogen, for example" , "person who's amusing ..." .)