I have a 'useful links' cloned group: Site field and URL field, I'd like to add a custom_html field with a link using the url field value to the group to view and check the value of url field from the meta box.
Hmm, I think it's not possible because the group is cloned and there is no way to attach multiple values of the clones into the meta box array. You can try JavaScript to update the HTML of the custom_html field when the site/url field is updated.