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All answers having "Intensifies" as clue:

3 Character Answers:

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RHO



4 Character Answers:

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AMPS



6 Character Answers:

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AMPSUP

BUILDS

REVSUP



7 Character Answers:

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DEEPENS

HEATSUP

LOUDENS

STEPSUP



8 Character Answers:

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ENHANCES

SHARPENS

THICKENS



9 Character Answers:

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ESCALATES

HEIGHTENS