I don't think Clemenza would have betrayed Mike, I dont think he would have had motive to... Whereas Pantangelli certainly did...

IMO The Rosato Brothers killed Clemenza.. and where trying to force Pantangelli out at well.. Pantangelli went to Mike for help, and Mike wouldnt help him. Frank was very loyal but got kicked around so much, even he had his limits.

"What's mine is not yours to give."