Using virtualbox is an excellent way to start with virtualization. Since there is no requirement for VT or other virtualization support, you can use it on nearly any ClearBOX. Moreover, since VMWare is no longer supported in ClearOS 6, This provides a great virtualization framework in nearly any condition.
Virtualbox requires the graphical environment. Install KDE or other Desktop for ClearOS.
VirtualBox requires libSDL and kernel sources in order to compile some specifics. Install the SDL package:
yum --enablerepo=clearos-core install SDL kernel-headers kernel-devel gcc
If wget is not installed, install wget.
yum install wget
Download the VirtualBox RPM
yum --enablerepo=clearos-core install VirtualBox-4.2-4.2.18_88780_el6-1.x86_64.rpm
Now that VirtualBox is installed, it should appear in your graphical desktop's start menu system. Click on the ClearOS Core icon » System » Virtual Machine.
From here you should be able to follow the guide on the VirtualBox Documentation Website.
Lastly, have fun!