SQL> create or replace function return_clob return clob as
  2    c clob;
  3  begin
  4    c:=rpad('x',4000,' ');
  5    c:=c||rpad(' ',4000,' ');
  6    c:=c||'z';
  7    return c;
  8  end;
  9  /
SQL>
SQL> select return_clob x from dual;

X
-----------------------------------------------------------
x




                                                                            
                                                                            
                                                                            
                                                                            
                                                                            
                                                                            
                                            z