The most common and most likely answer to this crossword clue is the 7 plus 8 letter word ITALIAN CONCERTO
I've seen this crossword clue in Financial Times
I
T
A
L
I
A
N
C
O
N
C
E
R
T
O
Buddy explains!
The straight part
We know that "suite by Bach" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case suite by bach = italian concerto.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.