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All answers having "Rears" as clue:

4 Character Answers:

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AFTS



5 Character Answers:

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ARSES

DUFFS

RUMPS



6 Character Answers:

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ERECTS

RAISES

STERNS

TUSHES



7 Character Answers:

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HEINIES

NURSLES



8 Character Answers:

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CABOOSES

KEISTERS

TAILENDS