make a excerpt on data from a meta box?

by

You could make use of wp_trim_words:

<p><?php
    echo wp_trim_words(
        get_post_meta( $post->ID, 'twpb_news_textnews', true ),
        55,
        '[&hellip;]'
    );
?></p>

Or, if you want the filters applicable to the regular excerpts to be used as well, write your own wrapper for it:

function wpse115106_news_excerpt( $text="" ) {
    $excerpt_length = apply_filters( 'excerpt_length', 55 );
    $excerpt_more = apply_filters( 'excerpt_more', ' ' . '[&hellip;]' );

    return wp_trim_words( $text, $excerpt_length, $excerpt_more );
}

and then use

<p><?php
    echo wpse115106_news_excerpt( get_post_meta( $post->ID, 'twpb_news_textnews', true ) );
?></p>

in your loop.