Clemenza was the most loyal of Michael's allies and subordinates. If his death had been due to foul play, Michael would have known about it and whacked the perps. Frankie was in the midst of a dispute with the Rosato brothers, and was at the party to lobby Michael to take his side. As we saw later, Frankie claimed that Clemenza "hated them Rosatos even more than I do." So, I guess that Cici was using a figure of speech--"that was no heart attack"--to imply that the aggravation over the Rosato brothers hastened Clemenza's demise.