Sorry guys, I just found the solution:
SELECT p.post_name, t.name as clientName
FROM $wpdb->posts AS p
INNER JOIN $wpdb->term_relationships AS tr ON ('p p.ID'ID = tr.object_id)
INNER JOIN $wpdb->term_taxonomy AS tt ON (tr.term_taxonomy_id = tt.term_taxonomy_id)
INNER JOIN $wpdb->terms AS t ON (t.term_id = tt.term_id)
WHERE p.post_status = 'publish'
AND p.post_type = 'portfolio'
AND tt.taxonomy = 'clients'
ORDER BY p.post_date DESC
I just don't have to define an alias for $wpdb->terms
in FROM
clause.