The term "clustering" does not have a well-defined meaning when used in reference to streaming servers, so this depends on the type of clustering to which you are referring. The three most common meanings include:
Single sign-on / distributed provisioning
This is used if you have a large number of users and want to spread them out across multiple physical servers.
Centova Cast v3 fully supports distributed provisioning through the use of a single web interface server which controls multiple back-end stream hosting servers.
With v3, your Centova Cast web interface can run on one machine anywhere in the world, and your streams can be hosted on one or more completely separate servers anywhere else in the world.
All stream management is performed through the single web interface server, providing a single, unified management interface for yourself and your customers.
Additional servers can be added at any time; after you install your web interface you can add additional servers by following the control daemon installation instructions in the installation manual.
This is used if you have one or more high-traffic streams which cannot entirely be served from a single physical server.
In this scenario, you would provision multiple physical servers which act as "relays" for the ShoutCast stream. Each relay contacts the ShoutCast stream and acts as a "repeater", rebroadcasting the stream to reduce the load on the main physical server.
Centova Cast does fully support relaying, however you will need to setup Centova Cast on each of your servers and manually configure each of your relays (this is not done automatically).
High-availability / fault-tolerant clustering
This is "true" clustering in the traditional sense, and is used when stream downtime must be kept to an absolute minimum.
High-availability (HA) clustering solutions are extremely complex, usually depend on custom hardware configurations, and always require installation and ongoing maintenance by a qualified systems administrator who is thoroughly familiar with HA clustering solutions. They require shared storage (NAS, GFS, drbd, etc.), a heartbeat solution, a replicated MySQL database, and many other elements that are not supported "out-of-the-box" by Centova Cast or any other ShoutCast control panel due to the complexity and custom nature of such solutions.
That said, it is certainly possible to use Centova Cast in such a scenario -- for the most part, if your systems administrator is familiar with HA clustering of nginx/PHP/MySQL, the same principles will apply to Centova Cast.