If you have already paid (in a paid edition) or registered, it won't let you use "clear and start over."   You can still edit everything, you just can't start from scratch.  

If you feel strongly that you need to start over, then to start from scratch you'd have to use a different account (User ID) or move to a desktop product.  But be aware that having multiple accounts (User IDs) could create logistical problems in subsequent years in keeping track of returns, transferring data from one year to the next, etc.