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All answers having "Off-key" as clue:

4 Character Answers:

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FLAT

SOUR



5 Character Answers:

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SHARP



6 Character Answers:

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ATONAL



7 Character Answers:

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UNTUNED



8 Character Answers:

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MISTUNED



9 Character Answers:

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OUTOFTUNE



10 Character Answers:

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DISCORDANT