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All answers having "See 51 Across." as clue:

3 Character Answers:

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IOO



4 Character Answers:

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QUOI

REST

ROLE

RULE

USSR



5 Character Answers:

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MAHAL



6 Character Answers:

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SCREAM



7 Character Answers:

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REDDING



16 Character Answers:

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OFHUMANHAPPINESS