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All answers having "Arena" as clue:

3 Character Answers:

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PIT



4 Character Answers:

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OVAL

RING

RINK



5 Character Answers:

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LISTS

PITON

REALM



6 Character Answers:

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ENDING

SPHERE



7 Character Answers:

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BESTIAL

ESTUARY

STADIUM



10 Character Answers:

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HIPPODROME



11 Character Answers:

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SANDINSPAIN



12 Character Answers:

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AMPHITHEATRE