ClearPass doesn't run on CentOS and users only have access to the ClearPass configuration. There may be components of CentOS or other OS flavours, but is has been hardened, modified and tuned in a way that it no longer can be considered a version number of CentOS.
If you are asking this for security reasons, you should check the Aruba Security Advisories on the Aruba website for information on vulnerabilities in ClearPass itself or one of the used third party components used in ClearPass.
ClearPass 6.6 is end-of-support, so you probably should upgrade if that is your question.