Use dbms_lob.writeappend to append value to clob type value
SQL> SQL> declare 2 l_clob clob := '12345'; 3 begin 4 dbms_lob.writeappend( l_clob, 5, 'ABCDEFGHIJK' ); 5 dbms_output.put_line( l_clob ); 6 end; 7 / 12345ABCDE PL/SQL procedure successfully completed. SQL> --