The Duke (1979)

As professional boxer Oscar "Duke" Ramsey approached middle age, he decided it might be a good idea to quit the fight game and open a restaurant in his native Chicago. When Benny, Duke's partner in Duke & Benny's Corner, is murdered, Duke decides, against the wishes of Chicago police detective Mick O'Brien, to track down the killer himself. After bringing the murderer to justice, Duke decides to become a private investigator.