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All answers having "Undo-" as clue:

3 Character Answers:

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DIS



4 Character Answers:

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OPEN

RUIN



5 Character Answers:

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ANNUL

ERASE



6 Character Answers:

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CANCEL

NEGATE

REPEAL



7 Character Answers:

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DESTROY

NULLIFY

REVERSE



8 Character Answers:

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UNBUCKLE