SELECT cln.id, o.*
  FROM orders AS o
       INNER JOIN metaorders AS m ON o.metaorder_id = m.id
       # LEFT JOIN clients cln ON m.client_id = cln.id
       LEFT JOIN (
                    SELECT id
                      FROM clients AS c
                 ) AS cln ON m.client_id = cln.id
       LEFT JOIN addresses AS addr ON cln.id = addr.client_id
WHERE 
      (cln.id IS NULL AND 
       EXISTS (SELECT 1 FROM order_products AS pdt WHERE (o.id = pdt.order_id) AND (pdt.product_id = 644))
      );

LEFT JOIN (SELECT ...

LEFT JOIN clients