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All answers having "Happened" as clue:

3 Character Answers:

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OCC

WAS



4 Character Answers:

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ASIT

ZING



5 Character Answers:

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AROSE

FARED



6 Character Answers:

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BECAME

BEFELL

GONEON



7 Character Answers:

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CHANCED



8 Character Answers:

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CAMETOBE

CAMETRUE

OCCURRED



9 Character Answers:

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CAMEABOUT



10 Character Answers:

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CAMETOPASS