Will Anderson

If you do that, the “more” links from archive pages will point to an invalid anchor. Is that what you want?

But there is no the_content_more_link filter when viewing a single post. Better, filter the_content and add a unique class to the “more” span using a RegEx replace.

function add_more_span_class( $content ) {
    $content = preg_replace('/(more-[0-9]+)/', '$1" class="more-span', $content);
    return $content;
}
add_filter( 'the_content', 'add_more_span_class');

Then you can add CSS to make that span clear.

.more-span {
    display: block;
    clear: both;
}