Okay. Except for the FBI chart at the Senate hearing, there is no mention of a Corleone underboss in either the novel or the films. Neither Clemenza or Pentangeli was an underboss. When Michael moved to Nevada, he was still the don. However, both Clemenza and Pentangeli were dons also of the Corleone family in NY. In fact, Pentangeli tells Mike that he wants to run his family without you on my back. Of course, Mike retorts that his family is still called Corleone. There are analogous situations in busines where the founder of a company owns plenty of that company's stock and exercises considerable influence, but the company is run by a president or CEO. Also, it's similar to the role of Sam Giancana (and a succession of others) in Chicago in relation to Tony Accardo who, though seemingly retired, was the power behind the throne for a very long time.

Cicci was probably Frankie's most reliable button man to whom he gave alot of latitude to run the NY family's street action without giving him a formal title.