Run str_replace on title and save the output to a custom field

the_title_rss() outputs its value straight away so it can’t be used in a function like that, you would have to use get_the_title_rss()

Or alternatively the_title_rss() has a filter we can hook in to and return your shortened title is possible. (You would add this to your functions.php)

function gg_short_title_rss($title)
{
    // This can return false, so check there is something
    $short_title = substr($title, 0, strpos($title, ' –'));
    if ($short_title) {
        return $short_title;
    }

    // Else just return the normal title
    return $title;
}
add_filter('the_title_rss', 'gg_short_title_rss', 10, 1);