This mission is given to you by Lester ("L" on your map) and you must finish the previous assassination missions before this one is available, as well as I Fought the Law... He wants you to kill Jackson Skinner, who happens to have a thing for working girls, and there's a regular he picks up in La Puerta.
Head to La Puerta and find where the girl is located. Park across the street and now you will need to wait until she enters the target's car, which will not happen right away. When she does get in a vehicle, kill the driver and get away from the scene to finish the mission. If you have a sniper rifle, you can hide somewhere close by and get an easier shot on the target.