Oxygen, for instance (3)
I believe the answer is:
gas
'oxygen for instance' 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 "Something that is neither solid not liquid" , "THIRD PART OF 23" , "Eg, nitrogen" , "Fumes, vapour" , "Carbon dioxide, say" .)