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.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.