function zfly9_custom_excerpt( $content ) {
    if ( ! is_singular() ) {
        $content = wp_trim_words( $content, 55 ) . ' <a class="more-link" href="' . esc_url( get_permalink() ) . '">Read more!</a>';
    }

    return $content;
}
add_filter( 'the_content', 'zfly9_custom_excerpt' );