The Mob (1951)

Police detective Damico, outwitted by mob killer Blackie Clay, is nominally suspended; actually he goes undercover (as Tim Flynn, ex-con longshoreman) to find Clay and expose the waterfront rackets. In character, Damico throws his weight around so much that the mobsters try to get rid of him; surviving this, he begins to realize that few of those around him are what they seem.