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All answers having "Improvement" as clue:

4 Character Answers:

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GAIN



6 Character Answers:

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MEJORA

REFORM

UPLIFT

UPTURN



7 Character Answers:

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UPGRADE

UPSWING



8 Character Answers:

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PROGRESS



10 Character Answers:

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BETTERMENT

REFINEMENT



11 Character Answers:

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ENHANCEMENT



12 Character Answers:

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AMELIORATION