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All answers having "Kook" as clue:

3 Character Answers:

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NUT

RHO



4 Character Answers:

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LOON

YOYO



5 Character Answers:

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FLAKE

LOONY

NUTSO

WACKO



6 Character Answers:

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MADMAN

NUTJOB

WEIRDO



7 Character Answers:

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LUNATIC

NUTCASE

ODDBALL



8 Character Answers:

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CAACKPOT

CRACKPOT



9 Character Answers:

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DINGALING

SCREWBALL