"Clean Slate" simply means a fresh installation on blank partitions. Or even better, with no partitions at all (creating fresh ones). So if you really mean to start on a "clean slate", boot into live session, delete all partitions with Gparted and create a fresh partition table (Device > Create Partition Table..) and create fresh partitions on it. Then retry the installation.
If you had configured RAID, make sure to reset or disable it (I don't have much idea about it, you should know better if you used it). Make sure to erase its metadata from the HDD as well, which usually means -
..replace "sda" with the target drive (sdb, sdc..)
sudo dmraid -E -r /dev/sda