Marcelo wants you to deal with a traitor called Flavio, formerly of the Assassinos, who is going to testify in court against the gang, in return for his freedom. We have to take him out. From your starting position, wait around 10 seconds or so, then you will see Flavio being driven past with a police escort. Drive up along side him and take him out. He may well get out of the car and try to run, but you cannot let him get away.
Once you have dealt with him, you will have four wanted shields - which means you will have armored police vehicles to contend with. Try to get a motorbike or fast vehicle and locate at least one police shield on the map to take you down to three - this makes it significantly easier to evade them.
Once you have achieved that, the mission will be over. Note that Marcelo insults Raul in the post-mission cut scene, which isn't likely to go down well with Angel.
Reward: 5,000R$