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"informer" is also used as a crossword clue. If that's what you saw, the answer is often rat (3 letters).
Definitions
one who reveals confidential information to the police or other authority
Word origin
Inherited from Middle English enformour, from Anglo-Norman enfourmour; equivalent to inform + -er.
Used in a sentence
“Being an informer often meant living in fear.”
“The police relied on an informer to catch the gang.”
“He acted as an informer during the rebellion.”
Source: Wiktionary, CC BY-SA 4.0
Used as a crossword answer1 curated clues
01“Secret tipster”8 letters
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