Complex sentence: Chicago's most powerful gangster, who sometimes calls on Jack and Sammy to protect him from other crime lords; they do so grudgingly, though Capone's money can be a very powerful incentive.
Split into simpler sentences: Chicago's most powerful gangster, who sometimes calls on Jack and Sammy to protect him from other crime lords. They do so grudgingly, though Capone's money can be a very powerful incentive.