Try this:
CREATE TABLE #T (
    ORDER_ID int,
    FS_CODE varchar(8)
);
INSERT INTO #T(ORDER_ID, FS_CODE)
    VALUES    (100, 'AAA'),
            (200, 'AAA'),
            (200, 'BBB'),
            (300, 'AAA'),
            (300, 'BBB'),
            (300, 'ZZZ'),
            (333, 'AAA'),
            (334, 'AAA'),
            (400, 'BBB'),
            (400, 'AAA');

WITH ORDERS_REWORKED AS (

    SELECT DISTINCT T.ORDER_ID, X.FS_CODES
    FROM #T AS T
        CROSS APPLY...