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All answers having "Screwball" as clue:

3 Character Answers:

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NUT



4 Character Answers:

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KOOK

LOON

WACK

ZANY



5 Character Answers:

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CRAZY

FLAKE

INANE

KOOKY

NUTTY

WACKO

WACKY



7 Character Answers:

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NUTCASE



8 Character Answers:

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CRACKPOT

HEADCASE



9 Character Answers:

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QUEERFISH

SENSELESS



10 Character Answers:

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OFFTHEWALL



15 Character Answers:

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WILDANDCRAZYGUY