Lung capacity provides opportunity to relax (9,5)
I believe the answer is:
breathing space
'lung capacity' is the definition.
The answer and definition can be both quantities as well as being singular nouns.
Perhaps they are linked in a way I don't understand?
'opportunity to relax' is the wordplay.
I cannot really see how this works, but
'opportunity' could be 'in' (in can mean an opportunity or way into something) and 'in' is present in the answer.
This may be the basis of the clue (or it may be nonsense).
'provides' is the link.
Can you help me to learn more?
(Other definitions for breathing space that I've seen before include "might give one an opportunity to relax" , "relief" , "room" , "Brief respite" , "chance to relax" .)