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All answers having "Effective" as clue:

4 Character Answers:

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ABLE



5 Character Answers:

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VALID



6 Character Answers:

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COGENT

POTENT

USEFUL



7 Character Answers:

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INFORCE

OPERANT

TELLING



8 Character Answers:

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FORCEFUL

FORCIBLE



11 Character Answers:

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EFFICACIOUS