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All answers having "Numbskulls" as clue:

4 Character Answers:

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SAPS



5 Character Answers:

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BOOBS

BOZOS

DODOS

DOLTS

DOPES



6 Character Answers:

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IDIOTS

MORONS



11 Character Answers:

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CHEESEHEADS



12 Character Answers:

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BUBBLEBRAINS