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All answers having "Guess" as clue:

3 Character Answers:

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EST

WHO



4 Character Answers:

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IDEA

READ

SHOT

STAB

WHAT



5 Character Answers:

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HUNCH

OPINE



6 Character Answers:

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ASSUME

RECKON



7 Character Answers:

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IMAGINE

PREDICT

SURMISE



8 Character Answers:

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ESTIMATE



9 Character Answers:

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TAKEASTAB



12 Character Answers:

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WHOGOESTHERE