The most common and most likely answer to this crossword clue is the 11 letter word KETTLE-DRUM
I've seen this crossword clue in Financial Times
K
E
T
T
L
E
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D
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M
Buddy explains!
The straight part
We know that "a musical instrument" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case a musical instrument = kettle-drum.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 KETTLE-DRUM mean?
KETTLE-DRUM noun- A large hemispherical brass percussion instrument (one of the timpani) with a drumhead that can be tuned by adjusting its tension.
- An informal social party at which a light collation is offered, held in the afternoon or early evening.