Grand Theft Auto III/Cutting The Grass

Brief
There's a SPANK factory run by the Cartel somewhere in Liberty City, but the Mafia doesn't know where, and their secret plans to curb the Columbians are always in vain, as the Cartel seem to know the Mafia's every step. There's a rat in their midst, and Salvatore suspects Curly Bob, a guy working at Luigi's club, based on his suspicious income, so he instructs Claude to follow him, and, if he is talking, to silence him.

Walkthrough
Get to the Club as soon as possible, as Curly Bob won't be there forever. Park you vehicle near the pavement, facing Chinatown. Soon Curly Bob will get into his taxi and will go to the harbour. Follow him, but not too closely or he'll get spooked.

Alternatively, you can jack his taxi before he gets in, or park another taxi that is closer to the club than the one waiting. Once Bob is in your taxi, you'll be instructed to drive him to the harbour (and you'll earn a Social Club achievement in Definitive Edition).

Once at the harbour, you'll find that Bob's been passing Mafia secrets to not only the Cartel, but also Claude's slimy ex-girlfriend Catalina. Once he's finished talking, you've seen enough. Run him over with your vehicle. Make quick work out of it, as he's got a shotgun that can wreck your ride in a hurry.