One of the design decisions in Clearance is the lack of a username. Clearance dictates an email address & password are used for authentication.
Your email address is a unique identifier. Requiring users to create an additional username for authentication is inviting the following headaches:
Not all web apps using authentication need a username, but some do. Consider Twitter: your username is a crucial part of the system.
While Clearance will never include the username feature, it is flexible enough for you to add this feature to your app.
Here are the general steps:
Mihai Anca wrote an example app & maintains it at ropiku/clearance-username