The most common and most likely answer to this crossword clue is the 5 plus 5 letter word COUNT BASIE
I've seen this crossword clue in Financial Times
C
O
U
N
T
B
A
S
I
E
More clues leading to the result COUNT BASIE
Buddy explains!

The straight part
We know that "jazzman" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case jazzman = count basie.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.