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All answers having "Assembles" as clue:

4 Character Answers:

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RIGS

SITS



5 Character Answers:

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HERDS

MAKES

MEETS



6 Character Answers:

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ERECTS

PIECES

RIGSUP

SETSUP



7 Character Answers:

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GATHERS

RALLIES



8 Character Answers:

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COLLECTS

CONVENES



15 Character Answers:

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GATHERSTOGETHER