Ah, my bad. "BATTLEGROUND" was removed a long time ago. All instances use "INSTANCE_CHAT" instead.
Code:
/run SendChatMessage(string.format("{Triangle} %s {Triangle} - CLEAR",GetMinimapZoneText()),select(2,GetInstanceInfo())=="pvp" and "INSTANCE_CHAT" or IsInRaid() and "RAID" or "YELL")