Cryptic crossword clue
Old instrument gets bird changing key 7

The most common and most likely answer to this crossword clue is the 7 letter word CITTERN

I've seen this crossword clue in The Guardian
C
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T
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Buddy explains!

The straight part
We know that "Old instrument" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case old instrument = cittern.
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 CITTERN mean?

CITTERN noun
  1. A stringed instrument (chordophone), played with a plectrum (a pick), and most commonly possessing four wire strings and chromatic frets.

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