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All answers having "Insincere" as clue:

3 Character Answers:

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LIP



4 Character Answers:

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GLIB



5 Character Answers:

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EMPTY

FALSE

PHONY



6 Character Answers:

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HOLLOW

PHONEY

UNFELT



7 Character Answers:

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UNMEANT



8 Character Answers:

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AFFECTED

TWOFACED



12 Character Answers:

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DISINGENUOUS

HYPOCRITICAL

MEALYMOUTHED



13 Character Answers:

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TONGUEINCHEEK