The most common and most likely answer to this crossword clue is the 4 plus 4 plus 6 letter word ELLA FITZ GERALD
I've seen this crossword clue in Financial Times
E
L
L
A
F
I
T
Z
G
E
R
A
L
D
Buddy explains!
The straight part
We know that "jazz singer" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case jazz singer = ella fitz gerald.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.