Hi Wyme62,
If you are talking about the @icloud.com address created when the account was first opened (the 'primary address', and shown at the top of the list as the 'iCloudIMAP address' in Preferences>Accounts at icloud.com's Mail page) you cannot delete it. You can only delete 'email aliases' - additional addresses added later.
You should be aware before you start that once you've created an alias you cannot turn that address into a full iCloud account or move it to another account.
More information on aliases here: Add and manage email aliases for iCloud Mail on iCloud.com - Apple Support
Turn off, turn on, or delete an alias
You can turn an email alias on or off at any time. You can also delete email aliases that you no longer want to use. While turning off an email alias is temporary, deleting it is permanent. When an alias is turned off or deleted, email sent to that alias is returned to the sender.
1) In Mail on iCloud.com, click
at the top of the Mailboxes list, then choose Preferences.
2) Click Accounts, select an iCloud Mail alias in the list, then do any of the following:
*Turn off an alias: Select “Disable alias.”
*Turn on an alias: Deselect “Disable alias.”
*Delete an alias: Click “Delete alias,” then click Delete.
3) Click Done.
Please do not hesitate to reach back out to us if you have questions or need any additional support.
Thanks for using the Apple Support Communities.
Cheers!