I always felt like they were "teasing" Clemenza being the traitor with this line :

Don Corleone -- you once said that the day would come when Tessio and me could form our own Family. Til today, I would never think of it. I must ask your permission...

And Mike says not til they come back from Nevada.

Clemenza spoke up about leaving the family not Tessio and then Mike said NO.

Just my view of things