Respiratory disease (9)
I believe the answer is:
pneumonia
'respiratory disease' is the definition.
(I know that pneumonia is a type of respiratory disease)
This is the entire clue.
(Other definitions for pneumonia that I've seen before include "Piano menu? (anag)" , "Lung infection" , "Inflammation of the lungs" , "Respiratory disease" , "Inflammation of the lung" .)