What makes you think that Clemenza was the one who planted the gun? There was still plenty of time for Clemenza to have one of his people shoot over from the Corleone compound into the City to plant that gun.
Cause Michael said "If Clemenza can figure a way" to plant the gun. So I assumed this would not be something Clemenza would delegate, especially so close to the Paulie incident.
"Io sono stanco, sono imbigliato, and I wan't everyone here to know, there ain't gonna be no trouble from me..Don Corleone..Cicc' a port!"
