ALTER SESSION SET NLS_DATE_FORMAT = 'MMDDYYYY HH24:MI:SS';

CREATE table t(
seq_num integer  GENERATED BY DEFAULT AS IDENTITY (START WITH 1) NOT NULL,
c CLOB,
create_date DATE DEFAULT SYSDATE
);
/

insert into t (c) values (
      rpad('X',20,'X')
  );
/


CREATE PROCEDURE lob_append(
  p_clob IN OUT CLOB,
  p_text IN     VARCHAR2
)
AS
  l_text varchar2(32760);
BEGIN
  -- newline each time code is appended for clarity.
  l_text := chr(10)
            || p_text || chr(10)
            || '['||TO_CHAR (SYSDATE, 'MMDDYYYY HH24:MI:SS')||']'||chr(10);

  dbms_lob.writeappend(p_clob, length(l_text), l_text );
END;
/


DECLARE
  l_clob CLOB := empty_clob();
BEGIN
  SELECT c INTO l_clob FROM t WHERE seq_num = 1 FOR UPDATE;

  lob_append(l_clob, rpad('Z',20,'Z'));

l_clob  := empty_clob();
SELECT c INTO l_clob FROM t WHERE seq_num = 1 FOR UPDATE;

lob_append(l_clob, rpad('Y',10,'Y'));


END;
/

UPDATE T
   SET C = RPAD('Z',20,'Z') || CHR(10) || C
 WHERE SEQ_NUM = 1
/