Hungarian composer of the opera Bluebeard's Castle (6)
I believe the answer is:
bartok
(Other definitions for bartok that I've seen before include "Bark to attract great Hungarian composer" , "Hungarian composer of folk music" , "Hungarian composer and pianist, d. 1945" , "Bela -, Hung. composer" , "Bela ------ (1881-1945), Hungarian composer" .)