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All answers having "Definitely not!" as clue:

4 Character Answers:

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NONO

OHNO



5 Character Answers:

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NEVER

NOSIR

NOWAY



6 Character Answers:

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HARDLY

NOFEAR



7 Character Answers:

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TITANIC



9 Character Answers:

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BYNOMEANS

INNOSENSE



15 Character Answers:

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NOTBYALONGSIGHT