After hearing Cici say that I figured he would have atleast gone into detail. Something like

Fredo:"Heart attack, huh?"

Cici:"No - No, that was no heart attack."

Fredo:"What do you mean? Everyone around here heard it was a heart attack."

Cici: "______"(real description of Clemenza's death)

Atleast that's how I imagined the convo would have gone had Frankie not started complaining about waiting for an audience with Michael.