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All answers having "Dumbfounded" as clue:

4 Character Answers:

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AGOG

AWED



5 Character Answers:

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AGAPE

AGASP

ATSEA



6 Character Answers:

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AGHAST

AMAZED



7 Character Answers:

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ATALOSS

STUNNED



9 Character Answers:

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AWESTRUCK

STAGGERED



10 Character Answers:

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SPEECHLESS



13 Character Answers:

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FLABBERGASTED

THUNDERSTRUCK