As both Clemenza'a and Tessio's loyalties were mainly with Vito I find it strange that people cannot envisage Clemenza also turning traitor.

It would have been years after Vito's death that Clemenza betrayal would have occured, so for me is even more believable than Tessio betraying the family on the day of Vito's funeral.

Also, Tom certainly thought that Clemenza had it in him to turn traitor - ' I always thought it would have been Clemenza'.