Return of hit song Fever (7)
I believe the answer is:
malaria
'fever' is the definition.
(I've seen this before)
'return of hit song' is the wordplay.
'return of' 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' reversed gives 'mal'.
'mal'+'aria'='MALARIA'
(Other definitions for malaria that I've seen before include "Illness" , "Tropical disease" , "Disease causing fever transmitted by mosquitos" , "It's transmitted by a mosquito bite" , "Tropical fever" .)