Making a comeback, hit song "Fever" (7)
I believe the answer is:
malaria
'fever' is the definition.
(I've seen this before)
'making a comeback hit song' is the wordplay.
'making a comeback' shows that the letters should be reversed in order.
'hit' becomes 'lam' (to 'lam' means to hit hard).
'song' becomes 'aria' (aria is a kind of song).
'lam' written backwards gives 'mal'.
'mal'+'aria'='MALARIA'
(Other definitions for malaria that I've seen before include "Fever caused by mosquito" , "Disease spread by mosquito bites" , "Tropical disease" , "Mosquito-transmitted disease" , "It's transmitted by a mosquito bite" .)