These commands are typically only necessary when you have decided to form a clan, if you are instead only interested in joining one or using the current feature, please see the post containing player commands.
/clan create [tag] [name]
/clan bb (msg) - Add a message to the bulletin board.
/clan war (start/end) [clan tag] - Start or end a war with a clan.
/clan ally (add/remove) [clan tag] - Add/remove a clan as an ally.
/clan rival (add/remove) [clan tag] - Add/remove a clan as a rival.
/clan modtag [tag] - Modify the tag of the clan.
/clan kick [player] - Kick a player from the clan.
/clan invite [player] - Invite a player to the clan.
/clan setrank [player] [rank] - Set player at specified rank.
/clan trust [player] - Set a member as trusted
/clan untrust [player] - Set member as untrusted.
/clan promote [player] - Promote a member to leader.
/clan demote [player] - Demote a leader to member.
/clan disband - Disband (entirely dremove) your clan.
Colour chart for Reference.
