Add a CSS class to a certain text field for node display
Another option would be to use Display Suite if you didn't want to use hooks in template.php.

This would allow you to add the class directly from the content type edit page, this would probably be a little easier to manage if you started adding more classes to other fields.


After downloading Display Suite, you would need to enable: Display Suite, Display Suite UI, Display Suite Extras.
Then go to /admin/structure/ds/list/extras and check Enable Field Templates
Go to your content types manage display page and use the "Select A Layout" dropdown, this will enable display suite for that view mode under that content type.
Arrange your fields as needed and click the gear button to the right of the field and select Field Template: Expert
Check "Field Item" or any of the wrappers and add a class and an element and you're done.


It may seem like a lot to do to complete your specific task but it will make your life much easier in the long run.  I haven't built a single site without display suite since I discovered it.