What we breathe (3)
I believe the answer is:
air
'what we breathe' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for air that I've seen before include "Tune - mood - atmosphere" , "Bearing; tune" , "Broadcast; mien" , "Mix of gases surrounding the earth" , "Substance breathed in" .)