Once you have registered your return, or paid for it using the Online edition of TurboTax, you are no longer able to use the Clear and Start Over feature.
What you can do is to delete the forms you have entered. To do this, go to My Account > Tools > Delete a Form. Start with the forms you have entered (such as W-2s) before you delete the worksheets.
If your intent is create a new tax return for someone else, this step will delete your tax return. To create a new return, you must create a new login. Otherwise, you will overwrite your return.
Please remember to download an electronic copy of your completed return. Every year we hear from people who are sorry they didn't do that. TurboTax Online only stores copies in the cloud.