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All answers having "Proper" as clue:

3 Character Answers:

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APT

DUE

FIT

RHO



4 Character Answers:

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JUST

MEET

NICE

NOUN

PRIM

TRUE



5 Character Answers:

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ETHAN

LEGIT

MORAL

RIGHT

STAID



6 Character Answers:

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DECENT

SEDATE

SEEMLY



7 Character Answers:

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CORRECT



8 Character Answers:

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DECOROUS

RIGHTFUL



9 Character Answers:

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DERIGUEUR