Note-writer's favourable report on Simpson Jr? (6)
I believe the answer is:
bartok
'note-writer's' is the definition.
(composer Bela Bartok, writer of musical notes)
'favourable report on simpson jr?' is the wordplay.
'favourable report' becomes 'ok' (calling something OK is reviewing it favourably).
'on' is a charade indicator (letters next to each other).
'simpson jr?' becomes 'bart' (Bart Simpson, son in The Simpsons).
'ok' put after 'bart' is 'BARTOK'.
(Other definitions for bartok that I've seen before include "Hungarian-born composer, d. 1945" , "Bela -, Hung. composer" , "Bark to attract great Hungarian composer" , "Bela ------ (1881-1945), Hungarian composer" , "Hungarian composer of folk music" .)