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All answers having "Aperture" as clue:

3 Character Answers:

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EYE



4 Character Answers:

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HOLE

PORE

SLIT

SLOT

VENT



5 Character Answers:

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CHINK

STOMA



6 Character Answers:

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FEISTY



7 Character Answers:

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OPENING

ORIFICE

PINHOLE