Clemenza plants the gun for Michael Corleone to use in the restaurant. In The Godfather, Peter Clemenza arranges for a concealed pistol to be placed in the bathroom before Michael meets Virgil Sollozzo and Captain McCluskey.
The meeting is designed as a trap: Sollozzo, who arranged the attack on Vito Corleone, wants to negotiate with Michael, while McCluskey provides police protection. Because Michael is searched before entering the restaurant, he cannot carry a weapon himself. Clemenza's plan gives him a chance to appear unarmed.
The pistol is hidden behind the toilet tank. Michael retrieves it, returns to the dining room, and kills both men before escaping. The scene is a turning point because Michael, previously presented as the son least interested in the family business, commits his first major murders and becomes a fugitive.
Some detailed accounts identify one of Clemenza's men as the person who physically places the gun, but Clemenza is the canonical answer because he organizes and oversees the operation.