Sweet mouse ill with flu in need of treatment (11)
I believe the answer is:
mellifluous
'sweet' is the definition.
('mellifluous' can be a synonym of 'sweet')
'mouse ill with flu in need of treatment' is the wordplay.
'with' means one lot of letters go next to another.
'in need of treatment' is an anagram indicator.
'mouse'+'ill'+'flu'='mouseillflu'
'mouseillflu' with letters rearranged gives 'MELLIFLUOUS'.
(Other definitions for mellifluous that I've seen before include "Smooth and musical" , "Sweet-sounding" , "Sweet, pleasing (voice)" , "Melodious" , "(Of a sound) honeyed" .)