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All answers having "Grim" as clue:

3 Character Answers:

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SAD



4 Character Answers:

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DIRE

DOUR

OGRE



5 Character Answers:

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BLEAK

DREAR

NOWIN

STARK

STERN



6 Character Answers:

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DISMAL



7 Character Answers:

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DIREFUL



10 Character Answers:

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DEPRESSING