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All answers having "Frightful" as clue:

4 Character Answers:

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DIRE

GRIM

UGLY



5 Character Answers:

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HAIRY

LURID

SCARY



6 Character Answers:

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GRISLY

HORRID



7 Character Answers:

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GHASTLY

HIDEOUS



8 Character Answers:

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ALARMING

HORRIFIC

TERRIBLE



9 Character Answers:

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STARTLING