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All answers having "Union ___" as clue:

3 Character Answers:

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CIO

REP

TIE



4 Character Answers:

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BLOC

DUES

SHOP



5 Character Answers:

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EATEN

FLYER

LABEL



6 Character Answers:

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FUSION

MERGER

SOVIET



7 Character Answers:

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ONENESS



8 Character Answers:

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ALLIANCE

MARRIAGE

MERGENCE