Clash is a Crucible match type.
Clash is a 6v6 mode, where points are gained through scoring kills against the opposing team. A standard kill grants a player's team 100 points, but certain types of kills grant bonus points (e.g. headshots). Level advantages are disabled in this mode.
Like all Crucible gametypes, group play is vital to success. Since there are no objectives, players are not penalized for low map control, and can bunker down together, or fan out as they see fit. Strength is numbers is paramount, as a lone player can be picked off very quickly by teamshooting enemies. Heavy Ammo is a key way to swing a losing game the opposite way, so controlling at least one box is vital, and trying to shut the opposing team down with the other is a large factor. Super usage, both offensively and defensively, is the main way to break deadlock moments where both sides cannot overtake the other.