Fumo informs us that the Syndicate and the Disciples have signed a pact and are coming to wipe out the Assassinos, starting at the nightclub. We need to go and sort them out. You will have a limited amount of time to get there, but it should be more than enough.
Once you arrive you will need to clear the outside area of the nightclub first, where there will be a handful of enemies to contend with. After that, you need to head inside.
Deal with the enemies in the main room, who will be there as soon as you enter the club, then head upstairs and into the balcony area. Once you trigger the waypoint, loads more enemies will enter the club. Use a rocket launcher if you can (but be aware of Fumo walking in front of you), otherwise, head in close and take them out with a machine gun. Even as you clear the first wave of enemies, more will enter en masse, so be prepared.
Once the area is secure, Fumo will nominate you as the new leader of the Assassinos.