Mogura no Uta

Tanibukuro city's Officer Kikukawa Reiji is the problem child of the police force. He doesn't always follow protocol and he enforces justice through his own personal morals. But it was that exact unwavering sense of righteousness that convinced the chief of police that Reiji was a perfect fit for a top secret undercover mission. He is given the task of infiltrating the Sukiya organization, the biggest and most dangerous mafia group of the Kanto region, and find solid evidence to arrest its leader Todoroki Shuho. But being accepted and trusted in the mafia means Reiji has to do unthinkable things and put himself in dangerous situations where one little slip and his life would be over.

(Source: MangaHelpers)