Much like a home-wrecker, a clique wrecker is a person who joins a clique and befriends everyone, then soon after, causes a big dramatic fight, making the original clique to hate each other.
Girl 1: Hey look, that clique wrecker Ralph is hanging out with the goth kids now.
Girl 2: Yeah. It's a shame, really. Within a week all those kids will hate each other because of her.