$args = array(
'posts_per_page' => get_option('posts_per_page'),
'post_type' => 'listing',
'tax_query' => array(
array(
'taxonomy' => 'listing_category',
'field' => 'slug', //can be set to ID
'terms' => array(81)
)
)
);
$query = new WP_Query( $args );