The most common and most likely answer to this crossword clue is the 6 plus 5 letter word BARREL ORGAN
I've seen this crossword clue in The Times
B
A
R
R
E
L
O
R
G
A
N
More clues leading to the result BARREL ORGAN
Buddy explains!

The straight part
We know that "other instrument" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case other instrument = barrel organ.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.