The most common and most likely answer to this crossword clue is the 2 plus 7 letter word EL CAPITAN
I've seen this crossword clue in The Times
E
L
C
A
P
I
T
A
N
Buddy explains!
The straight part
We know that "Operetta" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case operetta = el capitan.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.