EMPTY_CLOB() stores an empty CLOB
CREATE TABLE my_clob(
id INTEGER PRIMARY KEY,
clob_column CLOB NOT NULL
);
INSERT INTO my_clob (id, clob_column) VALUES (1, TO_CLOB('a string'));
INSERT INTO my_clob (id, clob_column) VALUES (2, TO_CLOB('asdf'););
UPDATE my_clob
SET clob_column = EMPTY_CLOB()
WHERE id = 1;