Neon or Freon (3)
I believe the answer is:
gas
'neon or freon' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for gas that I've seen before include "Hydrogen, for example" , "eg Helium" , "Hydrogen or oxygen, say" , "Gossip" , "Eg, oxygen" .)