Bela —, Hungarian composer of opera Duke Bluebeard's Castle and ballet The Wooden Prince (6)
I believe the answer is:
bartok
(Other definitions for bartok that I've seen before include "Bela -, nineteenth and twentieth century Hungarian composer" , "Bela ------ (1881-1945), Hungarian composer" , "Bela -, Hung. composer" , "classical composer" , "Hungarian composer and pianist, d. 1945" .)