You can add subClippboards under any Clippboard. To do so, mouse over your Clippboard in the menu on the left, then click the + that appears next to the Clippboard. Just name the new subClippboard, and it will appear alphabetically under the top level Clippboard with any other subClippboards. Click the arrow next to the top level Clippboard name to collapse and expand the subClippboards in the menu.
To change an existing top level Clippboard into a subClippboard, choose your Clippboard from the menu on the left, then click the Edit button at the top of the Clippboard, and select Edit. Use the Set as SubClippboard menu to choose the top level Clippboard you want this subClippboard to reside under. (You can also turn a subClippboard into a top level Clippboard. On the Edit page, scroll to the top of the Set as SubClippboard menu and select Top Level.)