In it's simplest form, clay is a method that is added to both ::oo::object and ::oo::class. These method allow access to structured data that is store inside of the object itself. Clay also provides mechanisms for method forwarding and delegation, as well as tools to script events during the mixin process.
The manual page is distributed as part of tcllib.
Clay has a wiki hosted in its fossil repository with entries on how to implement everything from a threaded event engine to using natural language processing to dispatch commands.
Clay is also the foundation for the httpd (Tcllib) module in Tcllib.