stool pigeon
US /ˈstuːl ˌpɪdʒ.ən/
UK /ˈstuːl ˌpɪdʒ.ən/
1.
a person who acts as a decoy or informer, especially for the police
:
•
The police used a stool pigeon to gather information on the gang.
•
He was accused of being a stool pigeon for the authorities.