Also, at the scene of Vito's funeral, the thought of Clemenza betraying Michael is brought up when Tom Hagen says, "I always thought it would have been Clemenza."
Michael doesn't disagree that Clemenza would be loyal until death, but that Tessio "was the smarter one" and cut a deal for himself first.
Clemenza also foreshadows possible disloyalty when he comes to Vito asking to break off into his own family (attempting to back-door Michael in the process). Vito shoots down that idea in a hurry, but still it's on the record that Clemenza was at least thinking about it.
I think Michael was being a little sarcastic about Tessio in that scene. When he says, "It's the smart move...", I think he means that it
looks bad for Michael and the Corleone Family because Michael was deceiving everyone into thinking he was weak and irresolute. But it was really a plan to get his enemies to show their hand--and Tessio did.
Yes, Clemenza did propose to break off and form his own family. But he prefaced it by saying, "Godfather, you once said that the day would come when me and Tessio could form our own families." So, it wasn't potential disloyalty so much as trying to redeem a pledge that Vito himself had made.
Far more revealing, IMO, is what came next:
Vito says, "Do you trust my judgment?" Clemenza says, fervently, "Always, Godfather." But Tessio merely hisses, "Yes-s-s-." And, if I remember correctly, Tessio simply leaves the conference without shaking hands or embracing anyone--a harbinger of disloyalty to come.
to shake with Michael before he leaves the room. I always saw this as a foreshadowing of his betrayal. Also, there is another significant part of that scene where Carlo thanks Vito for Michael's naming him as his 'right hand man" in Vegas. It shows that Carlo has totally bought into the idea that Michael is not really in charge and that he is as weak and clueless as Barzini has been telling him.