If you already registered in Free Edition, you cannot use "Clear and Start Over." You can still edit everything, however.
The only way to start with a fresh return would be to use another account (different User ID). But there are some drawbacks. With that approach you cannot transfer your 2017 info into your 2018 return, and would have to keep up with 2 accounts in the future, so it can lead to confusion later on.
The regular Free Edition is more limited this year due to tax law changes.
If you need any of the new Schedules 1-6, for instance, then you can't use Free Edition. Here are some common things that cannot be handled by the regular Free Edition:
Itemized deductions
Business or 1099-MISC income
Sales of stocks/mutual funds/bonds
Rental property income
Foreign Earned Income
HSA
Credits, deductions and income reported on the new schedules 1–6, such as the Student Loan Interest Deduction
TurboTax has another free product called the TurboTax Free File Program. It is free for both Federal and State--IF you meet just one of the qualifications. It can handle all the usual forms without the limitations of regular Free Edition.
See this FAQ to see if you meet one of the qualifications. Ignore the first two sentences; they no longer apply. It is up and running.
https://ttlc.intuit.com/questions/1900583-what-is-turbotax-free-file-programThe TurboTax Free File Program is located at the link below. It requires a new User ID from what you are currently using for Free Edition since it is a different website, and the IRS requires it to be disconnected from the TurboTax commercial products.
https://turbotax.intuit.com/taxfreedom/