Ecstatic composer getting flu treatment (8)
I believe the answer is:
blissful
'ecstatic' is the definition.
(I've seen this before)
'composer getting flu treatment' is the wordplay.
'composer' becomes 'bliss' (British composer Arthur Bliss).
'getting' means one lot of letters go next to another.
'treatment' indicates anagramming the letters (letters treated or processed differently).
'flu' with letters rearranged gives 'ful'.
'bliss'+'ful'='BLISSFUL'
(Other definitions for blissful that I've seen before include "Delighted" , "Happy, often delusively" , "Full of joy" , "Ecstatic, rapturously happy" , "Completely happy and contented" .)