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All answers having "Not serious" as clue:

4 Character Answers:

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IKID

MOCK



5 Character Answers:

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LIGHT

MINOR



6 Character Answers:

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JOKING



7 Character Answers:

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COMEDIC

JOCULAR

JOSHING

TRIVIAL



9 Character Answers:

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FACETIOUS



13 Character Answers:

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TONGUEINCHEEK