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All answers having "Loony" as clue:

3 Character Answers:

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MAD

NUT



4 Character Answers:

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BATS

DAFT

KOOK

LOCO

NUTS



5 Character Answers:

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BARMY

BATTY

DAFFY

DOTTY

NUTSO

WACKO



6 Character Answers:

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CUCKOO

INSANE

MADMAN

WEIRDO



7 Character Answers:

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NUTCASE



13 Character Answers:

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AROUNDTHEBEND



15 Character Answers:

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MADASAMARCHHARE