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All answers having "Heir" as clue:

3 Character Answers:

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REL

SON



4 Character Answers:

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WIFE



5 Character Answers:

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SCION

SPRIG



6 Character Answers:

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ELDEST



7 Character Answers:

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ALIENEE

HERITOR

LEGATEE



8 Character Answers:

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ALLOTTEE

ASSIGNEE

SURVIVOR



9 Character Answers:

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SUCCESSOR



11 Character Answers:

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MONARCHTOBE