The solution that @GenuinBeast48 mentioned is a good suggestion and I'd try that first. If that doesn't work, read #3 and #4 (which contains the above suggestion) in this thread and see if they help.
3) How can I delete my clan?
If you have other members in your clan, you can transfer ownership to them. You do this by selecting the member (in-game or on the mobile app) and change their role. Making them the Commander or Clan Leader will give them access to the clan. From that point on you should be able to leave the clan.
If you are the only member in the clan then you will need to either invite someone else into the clan and transfer ownership to them. Unfortunately there isn’t a way to leave a clan if you are the only member in it. You will need to contact @ATVIAssist and hope that they can help you with this until a better solution is presented.
4) Why can’t I leave my clan?
If you are the only clan member than see the topic of “How can I delete my clan” above. If you are not the leader then you can leave the clan a couple different ways. One is by accepting another clan’s invite (in-game or on the mobile app) or by applying to another person’s clan. On the mobile app you can leave the clan without joining another clan by clicking on the “Leave Clan” button (helmet with left arrow) at the top of the Clan screen.
On 11/7 there was supposedly a patch that allows you to leave your clan regardless of being the commander/clan leader.
Another method you can use to try and leave your clan is as follows: If you cannot leave via cod app go to your online multiplayer on your console/gaming platform and where it says press Y for friends hit that and hit right until it shows you in your clan tab, highlight yourself then hit A and select leave clan.