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All answers having "Parasite" as clue:

3 Character Answers:

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RHO



4 Character Answers:

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TICK



5 Character Answers:

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DRONE

LEECH

LOUSE

MOOCH

TOADY



6 Character Answers:

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SPONGE



7 Character Answers:

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SPONGER



8 Character Answers:

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HANGERON

TAPEWORM