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All answers having "Informer" as clue:

3 Character Answers:

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RAT

SPY



4 Character Answers:

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FINK

NARK



6 Character Answers:

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CANARY

SNITCH



7 Character Answers:

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RATFINK

STOOLIE



8 Character Answers:

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SQUEALER

TELLTALE



11 Character Answers:

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STOOLPIGEON



13 Character Answers:

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WHISTLEBLOWER