You may belong to several clans but only one can be your main clan.
Your main clan is the one displayed on the right hand side your Lounge Homepage and everywhere in your user info area (top section).
The below steps explain how to set you main clan so that people visiting your page can see it. Having the Main Clan link in your user info area is a quick way to get to your Clan Lounge page.
All you need to do is to click on "Edit Mode" of you user info area, then select the Clan you want to make the Main one and save.
Your Main Clan info will now appear in your user info area: