Hydrogen or helium (3)
I believe the answer is:
gas
This clue is a double definition.
'hydrogen' is the first definition.
(hydrogen is a gas)
'helium' is the second definition.
(I've seen this in another clue)
(Other definitions for gas that I've seen before include "Idle chat" , "Petrol (US)" , "eg CO2" , "What's needed to drive" , "natural resource" .)