Recursive Manager - Oracle 10g x 11g
If you want to find the people "under" CDLEADER = 21, then why do use START WITH US.CDLEADER IS NULL in your CONNECT BY? Just simply use START WITH US.CDLEADER = 21:

WITH ADRECURSIVEUSERLEADER 
AS
(
    SELECT US.CDLEADER AS CDLEADER
          ,US.CDUSER AS CDUSER
          ,US.NMUSER
          ,US.FGUSERENABLED
      FROM ADUSER US
     START WITH US.CDLEADER = 21         -- parent
   CONNECT BY PRIOR US.CDUSER = US.CDLEADER -- child / parent
)
SELECT * 
  FROM ADRECURSIVEUSERLEADER T
 WHERE 1 = 1;


SQL Fiddle