CREATE TABLE XMLISSUE
                ( 
                  xmltablecolumn clob
                 
                );

DECLARE 
 XMLCLOB clob;

BEGIN
 
   SELECT  XMLTYPE(
                   DBMS_XMLGEN.GETXML(q'{SELECT * FROM   whse_detail e WHERE  ROWNUM <= 10000 }')
                  ).getclobval()   as data1  INTO XMLCLOB 
          FROM   DUAL; 
          
         -- SELECT XMLCLOB FROM DUAL;
        --  DBMS_OUTPUT.PUT_LINE(XMLCLOB);
        INSERT INTO XMLISSUE(XMLTABLECOLUMN) VALUES(XMLCLOB);
          
 -- SELECT LENGTHB(XMLTABLECOLUMN) AS AASDF
--FROM XMLISSUE;
 
END;

 SELECT XMLTABLECOLUMN AS xmlclob
FROM XMLISSUE;

You can use functions in DBMS_LOB to find the length of a clob and substring it:

drop table t purge;
create table t (
  x clob
);

declare
  tmp clob;
  str varchar2(32767) := lpad('x', 32767, 'x');
begin
  dbms_lob.createtemporary(tmp,true);
  
  for i in 1 .. 200 loop
    dbms_lob.writeappend(tmp,length(str),str);
  end loop;
  
  insert into t values (tmp);
end;
/

select dbms_lob.getlength(x) from t;

DBMS_LOB.GETLENGTH(X)  
6,553,400  

select dbms_lob.getlength(dbms_lob.substr(x, 4000, 1)) from t;

DBMS_LOB.GETLENGTH(DBMS_LOB.SUBSTR(X,4000,1))  
4,000  

http://docs.oracle.com/cd/E11882_01/appdev.112/e40758/d_lob.htm#ARPLS600

But there's a problem with this approach. Dbms_lob.substr returns a varchar2. In SQL the maximum length of this is 4,000 bytes*. So if you attempt to return strings longer than this you get an error:

select dbms_lob.substr(x, 4001, 1) from t;

SQL Error: ORA-06502: PL/SQL: numeric or value error: character string buffer too small

The limit of varchar2 in PL/SQL is 32k. So you can extract larger strings, but still not the 5Mb you're looking for:

declare
  subs varchar2(32767);
  tclob clob;
begin
  select x
  into   tclob
  from   t;
  
  subs := dbms_lob.substr(tclob, 32767, 1);
      
  dbms_output.put_line('Clob length: ' || dbms_lob.getlength(subs));
end;
/

PL/SQL procedure successfully completed.
Clob length: 32767

declare
  subs  clob;
  tclob clob;
begin
  select x
  into   tclob
  from   t;

  subs := dbms_lob.substr(tclob, 32768, 1);
      
  dbms_output.put_line('Clob length: ' || dbms_lob.getlength(subs));
end;
/

PL/SQL procedure successfully completed.
Clob length:

So how to get around this?

Make repeated calls to dbms_lob.substr in PL/SQL!

declare
  len   pls_integer;
  tclob clob;
  subc  clob;
begin

  select x
  into   tclob
  from   t;
  
  for i in 0 .. 99 loop
    subc := subc || dbms_lob.substr(tclob, 32767, (32767*i) +1);
  end loop;
  
  dbms_output.put_line('Clob length: ' || dbms_lob.getlength(subc));
end;
/

PL/SQL procedure successfully completed.
Clob length: 3276700

Note: When processing clobs, the functions work on the number of characters. This is fine if you're sure the strings only use single byte characters. But if your text can contain multi-byte characters, fetching the substring of 5*1024^2 could exceed 5Mb. If 5Mb is a hard limit you could convert the clob to a blob. Then substr works on number of bytes. Be aware: if the last character is multi-byte, it may incorrectly chop it into two!

*In 12.1 this is increased to 32,767 if you're using extended data types.