O for breathable air! (6)
I believe the answer is:
oxygen
I believe this clue is a double definition.
'o' is the first definition.
(O is the chemical symbol for oxygen)
'breathable air' is the second definition.
'for' is the link.
(Other definitions for oxygen that I've seen before include "Gas discovered by Joseph Priestley, d. 1804" , "life support" , "Gas given off in photosynthesis" , "The second most common element on earth" , "Gas that combines with hydrogen to form water" .)