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All answers having "Split up" as clue:

4 Character Answers:

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PART

REND



5 Character Answers:

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APART

DIVVY

ENDIT

SEVER



6 Character Answers:

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DIVIDE

PARTED

SHARED



7 Character Answers:

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DEMERGE

DIVORCE



8 Character Answers:

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DIVORCED

SEPARATE



11 Character Answers:

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PARTCOMPANY