WordNet (r) 3.0 (2006):

plainclothesman
    n 1: a detective who wears civilian clothes on duty

Moby Thesaurus II by Grady Ward, 1.0:

28 Moby Thesaurus words for "plainclothesman":
   Bow Street runner, FBI, FBI agent, Federal, G-man, Secret Service,
   Sherlock, Sherlock Holmes, T-man, detective, dick, fed, hawkshaw,
   hotel detective, house detective, house dick, inquiry agent,
   investigator, narc, operative, police detective, private detective,
   private investigator, revenuer, sleuth, store detective, tec,
   treasury agent