"The Art of Fugue" composer (4)
I believe the answer is:
'the art of fugue composer' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for bach that I've seen before include "JS --, baroque composer" , "Relating to stars" , "Family of German composers" , "German organist and composer, d. 1750" , "J. S. --, baroque composer" .)