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All answers having "Decisive" as clue:

4 Character Answers:

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BOLD

SURE



5 Character Answers:

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FATAL

FINAL



7 Character Answers:

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CRUCIAL

FATEFUL

PIVOTAL

TELLING



8 Character Answers:

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CLEARCUT

CRITICAL



9 Character Answers:

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CLIMACTIC



10 Character Answers:

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RESOUNDING