The most common and most likely answer to this crossword clue is the 8 plus 10 letter word GOLDBERG VARIATIONS
I've seen this crossword clue in Independent
G
O
L
D
B
E
R
G
V
A
R
I
A
T
I
O
N
S
Buddy explains!
The straight part
We know that "set by Bach?" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case set by bach? = goldberg variations.The cryptic/wordplay part
In cryptic crossword there's often an wordplay/cryptic part as well.This part involves a more complex hint, such as an anagram, homophone, hidden word, etc.
Sadly, We don't have an explanation for this specific crossword clue yet.