Is there an input filter to add the class clearfix to all  tags?
If you are trying to add the "clearfix" class to all of your paragraph tags, sitewide, it would probably be more simple to apply the "clearfix" properties to your "p" tag in your CSS files.

So you'd find:

.clearfix { ... }


and add in your paragraph tag like so:

.clearfix, p { ... }


Or copy the properties to your custom CSS file if you are worried about overwriting your changes.