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All answers having "Rigorous" as clue:

4 Character Answers:

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HARD



5 Character Answers:

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EXACT

HARSH

SKILL

STERN

TENOR



6 Character Answers:

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SEVERE

STRICT



7 Character Answers:

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AUSTERE

DRASTIC

EPITAPH

INDEPTH



8 Character Answers:

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EXACTING

THOROUGH



9 Character Answers:

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STRINGENT



13 Character Answers:

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THOROUGHGOING