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All answers having "Cuckoo" as clue:

3 Character Answers:

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ANI

MAD



4 Character Answers:

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BATS

DAFT

GAGA

LOCO

LOON

NUTS



5 Character Answers:

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BATTY

CLOCK

CRAZY

DIPPY

INANE

LOONY

NUTSY

NUTTY

SILLY

WACKO



6 Character Answers:

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INSANE

SCREWY



7 Character Answers:

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BONKERS

NUTCASE



13 Character Answers:

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OFFONESROCKER



15 Character Answers:

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UTTERLYDEMENTED