-- Table to be updated
CREATE TABLE example_table
(id          NUMBER NOT NULL PRIMARY KEY,
 clob_data   CLOB,
 import_date TIMESTAMP);

-- Temporary table
CREATE GLOBAL TEMPORARY TABLE my_temp_table (
  id NUMBER) 
ON COMMIT DELETE ROWS;

-- Trigger to keep track of modified records
CREATE OR REPLACE TRIGGER trig_get_modified_recs
AFTER UPDATE ON example_table
FOR EACH ROW
BEGIN
  INSERT INTO my_temp_table
  VALUES (:NEW.id);
END;
/

-- Trigger to update the import_date to SYSDATE
CREATE OR REPLACE TRIGGER trig_update_ts
AFTER UPDATE OF clob_data ON example_table
BEGIN
  UPDATE example_date
  SET import_date = SYSDATE
  WHERE id IN (SELECT id FROM my_temp_table);
END;
/

ORA-25006: cannot specify this column in UPDATE OF clause
 *Cause:  Attempt to create a trigger on update of a column whose
          datatype is disallowed in the clause, such as LOB and
          nested table.
 *Action: Remove the UPDATE OF clause.

UPDATE OF clob_data 

SQL> CREATE OR REPLACE TRIGGER trig_get_modified_recs
  2  before UPDATE ON example_table
  3  FOR EACH ROW
  4  BEGIN
  5   :new.import_date := SYSDATE;
  6  END;
  7  /

Trigger created.

:NEW.import_date