@ Kid Arthur - The thread that you wish to be reopened is authored by the same user as authored the newer thread that was recently closed. As you stated that he has left your clan, that means even if the older thread is reopened, no one in your clan will be able to edit or maintain the opening posts.
As you are not the author of the thread you are requesting be reopened, the request to reopening should come from the clan owner or a deputy owner, if that is what they wish to happen.
However, it would seem that a better solution would be for the owner, or whomever they designate the task to, to post a new thread so that the opening content can be updated and maintained.