It appears that you can not set your clan tags using the Bungie Destiny app. In order to set your clan tag you need to 1. Access the clan page from the browser on your console then click set as clan tab next to the interact tab under the clan name. Or from your iphone or Android 2. Open your browser not the bungie app. Access the clan page then click set as clan tab next to the interact tab under the clan name. If the browser tries to send you through the app don't do it. You can use the app to do everything else except setting clan tags.