If you mean by "completely clean" that ALL previous data has been removed from everywhere, the process can be simple, just takes time. Download from the WD website their utility package Data LifeGuard. Check this web page for more complete instructions:https://wdc.custhelp.com/app/answers/detail/a_id/1211/session/L3RpbWUvMTM5MDg4NTE3Mi9zaWQvam81ZVd0TGw%3D
Personally, I prefer to use the "for DOS" option. Using it, you must burn the downloaded version to a CD, then boot from that CD to use the tools. What you should use is the "Zero Fill" option. WARNING!! This totally destroys ALL data on the HDD you're working with, so make VERY SURE you choose to destroy data on the correct HDD unit!
A Zero Fill take a long time - several hours for your HDD - because it writes zeroes to EVERY sector of the unit. This operation also triggers a built-in self-check process that ends ups substituting known-good sectors for any found to be weak or failed, so that you have a "perfect" HDD when it is finished.
After you have done the Zero Fill, the HDD is just like brand new from the factory. Then you can start a fresh Install of Windows.