Some of those in triathlon used up contents of lungs (3)
I believe the answer is:
air
'some of those in triathlon used up contents of lungs' is the definition.
I can't tell whether this defines the answer.
'some of' is the wordplay.
'some' becomes 'a'.
'of' becomes 'ir' (I am not sure about this - if you are sure you should believe this answer much more).
'a'+'ir'='AIR'
Can you help me to learn more?
(Other definitions for air that I've seen before include "Ambience" , "Expose" , "Atmosphere, manner" , "Of which 78% is nitrogen" , "Bearing; tune" .)