I have read the faqs and still cant figure this out. We are a ps3 clan that will compete on the xbox one and ps4. Most of us have linked psn accounts and xbox live accounts.
Once your clan is setup using the new site (clans.callofduty.com) then your clan is already configured for both platforms. All you need is members. A member can have their profile linked to Xbox (360 or One), Playstation (PS3 or PS4), and eventually PC. Thus, a member can play on any of those platforms and if they are in your clan they will help you out. There is no configuration you have to do to the actual clan to allow this aside from accepting members from any or all of the platforms.