stoolpigeon
/ˈstulˌpɪdʒən/
noun
- A person who acts as a decoy or informer, especially for the police, to trap or expose criminals.
- The detective used a stoolpigeon to get close to the drug dealers.
- In the old crime novels, the stoolpigeon always wore a wire and hoped not to get caught.
- He agreed to be a stoolpigeon in exchange for a lighter sentence.