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All answers having "Dunderhead" as clue:

3 Character Answers:

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APE

ASS

NIT

OAF

SAP



4 Character Answers:

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BOOB

BOZO

CLOD

DODO

DOLT

DOPE

FOOL

SIMP

TWIT



5 Character Answers:

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DUNCE

IDIOT

MORON

SCHMO



6 Character Answers:

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GALOOT

NITWIT



7 Character Answers:

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HALFWIT



8 Character Answers:

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NUMSKULL