I always thought Cicci meant that Clemenza had a heart attack because of all the bullshit going on in new york. The Rosato business was taking its toll on the old timer. If there was an ounce of belief that someone killed Clemenza in Michael's mind, I don't think Michael would turn a blind eye. Cicci is a street gangster and wants the Rosato's gone. In his eyes, they're back in new York being gangsters while Michael is Nevada making deals with Hymen Roth. That's why Cicci is taking orders instead of giving them. He may be good at the card game "war", but Michael is a master at Chess.