An Author-it library is a multi-user relational database, and Author-it is a content manager for its own content. Using Clearcase for an Author-it library would be like trying to check the Clearcase database into Clearcase. It would serve no purpose. Users can simply connect to the Author-it database and edit the information directly in Author-it without any need for an external manager like Clearcase. Author-it deals with security, checking/checkout, audits, tracking, tasks, etc.
If you did use Clearcase to check-in/check-out Author-it libraries, it would mean that only one user at a time can access the Author-it library. You would effectively turn a multi-user collaborative database into a single-user document. This will heavily restrict collaboration and eliminate many of the productivity gains achieved by working in a collaborative environment with Author-it.