Excerpt length: get first paragraph

Assuming your paragraphs are marked with <p> tags, which can be set in the ACF field options, the following should work:

function custom_field_excerpt() {
    global $post;
    $text = get_field('news');
    if ( '' != $text ) {
        $start = strpos($text, '<p>'); // Locate the first paragraph tag
        $end = strpos($text, '</p>', $start); // Locate the first paragraph closing tag
        $text = substr($text, $start, $end-$start+4); // Trim off everything after the closing paragraph tag
        $text = strip_shortcodes( $text );
        $text = apply_filters('the_content', $text);
        $text = str_replace(']]>', ']]>', $text);
    }
    return $text;
}

You could also pretty easily modify this to locate the first <br /> tag, if you happen to be storing your ACF data with <br />‘s instead of <p>‘s.