Old lady left opera song in illness (7)
I believe the answer is:
malaria
'illness' is the definition.
(malaria is a kind of illness)
'old lady left opera song' is the wordplay.
'old lady' becomes 'ma' (both can informally mean mother).
'left' becomes 'l' (common abbreviation).
'opera song' becomes 'aria' (type of song in an opera).
'ma'+'l'+'aria'='MALARIA'
'in' acts as a link.
(Other definitions for malaria that I've seen before include "Tropical fever" , "Mosquito-borne fever" , "Mosquito-transmitted disease" , "Disease spread by mosquitoes" , "Mosquito-borne disease" .)