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All answers having "Staying power" as clue:

4 Character Answers:

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GRIT

LEGS



5 Character Answers:

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ELITE

VIGOR



6 Character Answers:

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TENURE



7 Character Answers:

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ANTONYM

ASPIRIN

INERTIA

STAMINA



9 Character Answers:

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ENDURANCE

LONGEVITY



12 Character Answers:

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STICKABILITY