There is no existing plugin to create a clan application form, however what you're asking is basically the same thing as a contact form, the only difference being that your users will be submitting clan-specific info rather than sending you a personal email. This I can do, however I am still very basic, and it will be just plain table-based html. But before I can code one up for you, I'll need to know what information you want your users to submit when they apply to join a clan.