Cortez' right hand man, Gonzalez, gave away vital information concerning Tommy's deal to someone, and that someone organized the ambush. So Cortez wants you to silence Gonzalez' loose tongue, for good!
Cortez will provide you with a chainsaw and Tommy will have to use it to hack Gonzalez to pieces. Take Cortez' Cheetah parked right next to the yacht and drive to Gonzalez' penthouse behind the Malibu Club, then enter. Gonzalez will be there with two guards, Gonzalez will run away and head into the street, he will run towards the Malibu Club, you will have to run after him and when you reach him, finish him with the chainsaw.
You will get a two star wanted level; kill Gonzalez' goons who attack you and take your car. You now have to lose the wanted stars, either get the nearby police bribes or go to the closest Pay 'n' Spray. Once you have lost your wanted level, it's mission accomplished. After you visit Pay 'n' Spray, you have completed the mission.
HINT: To kill Gonzalez faster, use any gun instead of going after him (because the chainsaw won't allow for you to run fast). Shoot him first, then finish off the guards. Get in the car and get to the nearest Pay 'n' Spray.