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All answers having "Certain" as clue:

3 Character Answers:

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SYN



4 Character Answers:

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SOME

SURE

TRUE



5 Character Answers:

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BOUND

SNIDE



6 Character Answers:

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AMULET

DEGREE

SECURE



7 Character Answers:

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ASSURED

SHEIKHS

SYNONYM



8 Character Answers:

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DEFINITE

INTHEBAG

POSITIVE

SUREFIRE



9 Character Answers:

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CONVINCED

INNODOUBT



10 Character Answers:

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INEXORABLE

INFALLIBLE



11 Character Answers:

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BEYONDDOUBT

FOUNTAINPEN



12 Character Answers:

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WITHOUTDOUBT



13 Character Answers:

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IMPROBABILITY



16 Character Answers:

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INCONTROVERTIBLE