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All answers having "Stifle" as clue:

3 Character Answers:

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GAG

SHH



4 Character Answers:

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DAMP



5 Character Answers:

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CHOKE

QUASH



6 Character Answers:

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HOLDIN

ITSELF



7 Character Answers:

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REPRESS

SMOTHER



8 Character Answers:

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STRANGLE



9 Character Answers:

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SUFFOCATE