The Cname www if it is set to be part of the base URL is OK. It depends if the website uses www or non www. But it should be consistent to avoid duplicate contents. So it would still be a clean URL with or without. Consistency is a must for SEO point of view.
Yes clean url should not contain query strings and extensions like .
.php, etc. I remember configuring clean URL for one site before and my boss wants to remove the
Query strings are dynamic hence the content of page may change depending on the query. This is usual on ecommerce sites where searching of products is a vital function.
Also, it should not contain program extensions like
Examples of clean url would be: