How do I initialize a User Performance, to start with a clean slate?

You have 4 different initialized starting points. You can select one as follows:
From the HOME screen
Press [CATEGORY] to launch the “Performance Category Search”
“Bank/Favorite” = All
“Attribute” = All
“Main” tap “Init”

Select from one of the four Initialized options:
“Init Normal (AWM2)” — creates initialized program with a sample-based Part 1
“Init Normal (FM-X)” — creates initialized program with an FM-X based Part 1
“Init Drum (AWM2)” — creates initialized program with 73 Drum Elements C0~C6
“Multi/GM” — creates initialized 16-Part program for sequencing; Acoustic Piano in all Parts except Part 10 which is a DrumKit

Selecting one of these as a starting point creates a new Performance — you can store 640 User Bank Performances.
Your current User Bank Performances can be viewed, as follows:
From HOME
Press [CATEGORY]
Set the “Bank/Favorite” = User

Or you can manage them, as follows:
Press [UTILITY] > touch “Contents” > “Data Utility” > touch the Folder labeled “Performance”

Yes, but how can I take an already-existing Performance, which I no longer want, and clear it by initializing? I understand that I cannot delete it.

Yes, but how can I take an already-existing Performance, which I no longer want, and clear it by initializing? I understand that I cannot delete it.

? — You can definitely Delete Performances.

If you want to delete a User Performance you do so as follows:
Press [UTILITY] > touch “Contents” > “Data Utility” > tap the Folder named “Performance”
Here you can see all of your User Performances; you can see in the upper right corner how many out of 640 maximum, you have created.
You can sort them alphabetically by Name (or tap again to reverse the list)
You can view them in order in which they were made, Date (or tap again to reverse this order)
The lower right corner will indicate the different Pages of data you can scroll through…

When you see the Performance you want to DELETE, tap its box; a white cursor highlight surrounds the Name
A pop-in menu appears
Touch DELETE.

If you need to Delete a bunch of Performances, you can activate the “Job” box in the right corner… this allows you select multiple Performance (check mark) or to Select All, for removal.

If you want to create a new Performance see the previous answer.

Extra Credit
The Data Utility > Folders are where you have access to your various types of data while it is in internal memory. As you see above you can Rename or Delete any of your User Performances here. This “Performance” Folder is dynamic — meaning when you LOAD a different User File .X8U or .X7U, all the contents of this Folder will be rewritten. When attempt to Load another User File, you will see a warning messages informing you that loading the new User File will replace the current User Bank.

This is not a problem if you already have a File for the currently loaded data. The warning message is to remind you that if you changed any data, added new Performances, changed something about the existing data - you should probably SAVE that data before proceeding. But likely you have the data saved already - just go ahead.

Thank you very much! Couldn't find this in the manual. I appreciate it!