Modify raw SQL created by WP Query to order posts by custom wp_posts table column

functions.php

<?php

/**
 * Make sample fake query to show working example
 */
function motivast_wp_query() {

	new WP_Query(array(
		/**
		 * This line will not modify your query but we can use it to
		 * identify query later `$wp_query->get('orderby') === 'custom'`.
		 */
		'orderby' => 'custom'
	));
}

add_action( 'wp', 'motivast_wp_query' );

/**
 * Filters the ORDER BY clause of the query.
 *
 * @param string   $orderby  The ORDER BY clause of the query.
 * @param WP_Query $wp_query The WP_Query instance (passed by reference).
 *
 * @return string
 */
function motivast_orderby_custom_column( $orderby, $wp_query ) {

	/**
	 * If WP_Query has `custom` as orderby parameter replace SQL
	 */
	if( $wp_query->get('orderby') === 'custom' ) {
		return 'wp_posts.custom DESC';
	}

	return $orderby;

}

add_filter( 'posts_orderby', 'motivast_orderby_custom_column', 10, 2 );

ok, its working. but it should be $wp_query->get('orderby') since its normal parameter in wp_query

Done, thank you!