ClearPass has two 'firmware slots' such that you can always fall back to the previous version after an upgrade. You cannot delete firmware, but if you upgrade to a new major release, like 6.7, 7.0 or 8.0 whatever comes next after 6.6, the 6.5.0 slot will be replaced by that new version.
Also, it would not free up disk space to remove the old version.
Can you explain what you want to achieve by deleting the 6.5.0 partition?