Unfortunately you can no longer leave a clan as the last person in it. So you need to either get someone else to join your clan then promote them to founder so you can leave. Or create an account on one of the supported platforms, invite it to the clan then promote that account to founder then when you get that alt account done as a founder the account you are currently on will be able to leave.
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That's a freaking stupid mechanic for something that is highly regarded for clan/teamwork