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All answers having "Stamina" as clue:

3 Character Answers:

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PEP



4 Character Answers:

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GRIT

GUTS

LEGS

WIND



5 Character Answers:

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ASSET

VIGOR



6 Character Answers:

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ENERGY

METTLE



9 Character Answers:

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ENDURANCE



12 Character Answers:

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STAYINGPOWER