One once producing scores of cakes or loaves, not tons (4)
I believe the answer is:
bach
'one' is the definition.
Although both the answer and definition are singular nouns, I don't see how one could define the other.
I don't understand how the rest of the clue works.
Can you help me to learn more?
(Other definitions for bach that I've seen before include "Affectionate address" , "Goldberg Variations composer, d. 1750" , "J. S. -, Ger. composer" , "Johann Sebastian . . . ., great 18th century German composer" , "Family of German composers" .)