Johann Sebastian —, German composer of keyboard music collection The Well-Tempered Clavier (4)
I believe the answer is:
bach
(Other definitions for bach that I've seen before include "Affectionate address" , "Johann Sebastian . . . . , great composer (4)" , "German organist and composer, d. 1750" , "dear to the Welsh" , "German baroque composer, d. 1750" .)