Nouns

(n)fink, snitch, snitcher, stoolpigeon, stool pigeon, stoolie, sneak, sneaker, canarysomeone acting as an informer or decoy for the police

Verbs

(v)fink, scab, rat, blacklegtake the place of work of someone on strike
(v)confess, squeal, finkconfess to a punishable or reprehensible deed, usually under pressure