This is a Gravina Crime Family mission.
"The papers are saying that Big Sal's brother Tiny got charged with drug trafficking. Seeing that little shit go down would be hilarious, but you know how the Gravinas operate. The only witness has no protection. Save him from Mob killers to ensure he testifies."
Drive to the alley in Oyster Bay where you will find the witness cowering behind a Lassiter Series 75 Hollywood. Take out the two gangsters present and then take cover behind the limo with the witness and wait for the next wave.
They come in waves of one or two from three directions: right, left, and straight ahead. Their erratic behavior can often make them hard to hit; you can either "spray and pray" or wait until they stop to open fire on you and take a shot then. They also have a bad habit of getting in cover and just sitting there, never coming out or even trying to shoot at you. Try waiting to see if they eventually pop their heads up or you may need to exit your cover and take the fight to them.
All in all there are about 15 gangsters to kill; once you're done head to the safe zone to end the mission.