Mother left song fever (7)
I believe the answer is:
malaria
'fever' is the definition.
(I've seen this before)
'mother left song' is the wordplay.
'mother' becomes 'ma' (ma can mean mother).
'left' becomes 'l' (common abbreviation).
'song' becomes 'aria' (piece of music in an opera).
'ma'+'l'+'aria'='MALARIA'
(Other definitions for malaria that I've seen before include "Disease from a mosquito bite" , "Mosquito-borne disease" , "It's transmitted by a mosquito bite" , "Protozoal disease" , "Mosquito-borne fever" .)