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All answers having "Obstructs" as clue:

4 Character Answers:

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BARS

DAMS

JAMS



5 Character Answers:

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BALKS

CLOGS

STOPS



6 Character Answers:

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BLOCKS

ESTOPS

JAMSUP



7 Character Answers:

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ARRESTS

CLOGSUP

HINDERS

IMPEDES

INVADER

OPPOSES

STYMIES