Composer Johann Sebastian __ (4)
I believe the answer is:
bach
'composer johann sebastian' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for bach that I've seen before include "musician" , "musical great" , "Goldberg Variations composer, d. 1750" , "German composer (affectionate to a Welshman?)" , "German composer, who had twenty children, d.1750" .)