I don't think that Clemenza was ever a Don in the movie. I think he was a caporegime under Michael, running New York on a long leash. Pentangeli inherited his position, and a shorter leash.

Ntra la porta tua lu sangu � sparsu,
E nun me mporta si ce muoru accisu...
E s'iddu muoru e vaju mparadisu
Si nun ce truovo a ttia, mancu ce trasu.