[1] : WordNet (r) 2.0

fink
     n : someone acting as an informer or decoy for the police syn:
         snitch, snitcher, stoolpigeon, stoolie, sneak,
         sneaker, canary
see also:
snitch snitcher stoolpigeon stoolie sneak sneaker 
canary 

[2] : Moby Thesaurus II by Grady Ward, 1.0

41 moby thesaurus words for "fink":
   bear witness against, betray, betrayer, blab, blabber, blabberer,
   blabbermouth, blackleg, blow the whistle, canary, delator,
   inform against, inform on, informer, narc, nark, peach, peacher,
   rat, scab, sell out, snitch, snitch on, snitcher, spy, squeal,
   squealer, stool, stool pigeon, stoolie, strikebreaker, talebearer,
   tattle, tattler, tattletale, tell on, telltale, testify against,
   tipster, turn informer, whistle-blower

Fink >> F Words
Fink, definition of term: Fink
fink_pag1.html