The most common and most likely answer to this crossword clue is the 9 letter word BOMBARDON
I've seen this crossword clue in Financial Times
B
O
M
B
A
R
D
O
N
Buddy explains!
The straight part
We know that "musical instrument" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case musical instrument = bombardon.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.
What does BOMBARDON mean?
BOMBARDON noun- A brass instrument, the bass version of the tuba.
- A bass instrument of the shawm family
- A organ pedal stop at 32' pitch which imitates the sound of the above mentioned instruments.