grass BrE ***
stool pigeon AmE

He didn't need a stool pigeon to tell him where they'd been.

He said the Government wanted him "to be a stool pigeon."

More likely, they were after the stool pigeon who was due there.

You'll be playing stool pigeon, right enough, but on this one job.

Slowly, the stool pigeon looked toward the floor beside the other table.

If one of his stool pigeons is on the job, we'd be wasting our time.

But now the stool pigeon's plight was a matter of the past.

You never can tell what some stool pigeon has heard.

These were the stool pigeons, the spies of the police.

There's still a long chance that some stool pigeon will locate him.