SQL Select query with postgresql
You where clause is incomplete and wouldn't work that way anyway. 

You need a subselect for this:

SELECT   *
FROM the_table
WHERE cli_nom in (select cli_nom 
                  from the_table
                  where CLI_ENSEIGNE = 'A' );


alternatively a co-related sub-select with an exists clause:

SELECT t1.*
FROM the_table t1
WHERE EXISTS (select 42
              from the_table t2
              where t2.cli_nom = t1.cli_nom
                and CLI_ENSEIGNE = 'A' );