Postresql using subquery select in order by clause

Is it possible to order by a selected columnname?

  SELECT * FROM tablex ORDER BY  quote_ident((   select column_name from information_schema.columns   where table_name = tablex AND column_name LIKE 'Index')) ASC; 

The output of this is not ordered according to the columnname Index.

So basically my question is if i can use a select query in the order by clause like this simple one:

SELECT * FROM tablex ORDER BY (SELECT Index FROM table WHERE....); 

What did i oversee?