SQL & PL/SQL

select u.u_id,'<?xml version="1.0" encoding="UTF-8"?>'||
xmlelement("Data",xmlelement("prod",d.p_name||'.'||t.t_name),
xmlelement("type",u.u_type),
xmlagg(xmlelement("alias",
--xmlelement("Name",ul.l_get),
xmlelement("rul",ul.postname),
xmlelement("type",ul.type_id )
))).getStringVal() AS "DATA"
from dev.prod u,dev.prod__prop ul,dev.prod_det d,dev.det t
where u.u_id=ul.u_id
and u.p_id=d.p_id
and d.d_id=t.d_id
and u.u_id=2169
group by u.u_id,d.p_name,t.t_name,u.u_type

I am getting the result set. but if I uncommented the commented line then I am getting the following error
ERROR at line 8:
ORA-06502: PL/SQL: numeric or value error: character string buffer too small
ORA-06512: at line 1
B'cas it is exceeding the characrter length 4000...
But CLOB should store N amount of data...Where I am doing wrong ....?
Thanks in advance..