The most common and most likely answer to this crossword clue is the 8 letter word MANDOLIN
I've seen this crossword clue in Independent
M
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More clues leading to the result MANDOLIN
- Instrument chap at party left at home
- Chap to kill carrying large instrument
- Chap, old, playing popular instrument
- Piece to perform on 3/4-size string instrument
- Chap wearing trousers playing old musical instrument
- Bloke playing old fashionable instrument
- Staff do nothing about instrument
- Bloke to swindle student getting popular instrument
- Old instrument with nothing left in time
- Chap going to party with large fashionable instrument
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 = mandolin.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 MANDOLIN mean?
MANDOLIN noun- A stringed instrument and a member of the lute family, having eight strings in four courses, frequently tuned as a violin, and with either a bowl-shaped back or a flat back
- A kitchen tool used for slicing vegetables (usually spelled mandoline)
- An RAF World War II code name for patrols to attack enemy railway transport and other ground targets