Ok, so you want to only display the latest featured post, right?
You need to specify everything in the WP_Query – there’s no need for additional checking, if you only want one post. Use the meta_query, like this:
$args = array (
'post_type' => 'nieuws',
'posts_per_page' => 1,
'orderby' => 'date',
'order' => 'DESC',
'meta_query' => array(
array(
'key' => 'published',
'compare' => '=',
'value' => 1
)
),
);
You can find more examples of such queries here: http://www.advancedcustomfields.com/resources/how-to/how-to-query-posts-filtered-by-custom-field-values/
