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All answers having "Certainly" as clue:

3 Character Answers:

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AYE

NOT

SIR

YES



4 Character Answers:

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SISI

SURE

VERY



5 Character Answers:

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CALYX

LEVER

NATCH



6 Character Answers:

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INDEED

OUIOUI

RIGHTO

SURELY

WHYYES

YOUBET



7 Character Answers:

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GESTAPO

NODOUBT

NOTHALF

SURREAL



8 Character Answers:

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OFATRUTH

TOBESURE



9 Character Answers:

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DOUBTLESS

FOSHIZZLE

SANSDOUTE



10 Character Answers:

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ABSOLUTELY

BYALLMEANS



11 Character Answers:

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BUTOFCOURSE

UNDOUBTEDLY



13 Character Answers:

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WORDBLINDNESS