create table tableA
(
id number,
encoded_description clob);

insert into tableA
(id, encoded_description)
values
(1, 'Zm9sbG93IHVw');

insert into tableA
(id, encoded_description)
values
(2, 'dG8gbWFueSByZWQgZmxhZ3M=');

commit;

Table A output which contains base64 encoded data in encoded_description field:

Table A:

    ID, encoded_description
    1   Zm9sbG93IHVw
    2   dG8gbWFueSByZWQgZmxhZ3M=



 create table tableB
    (
    id number,
    decoded_description clob);

Table B: output after conversion

    ID, Decoded_description
    1   <<Original Text>>
    2   <<Original Text>>