Oracle 18c

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CREATE TABLE t (id NUMBER, c CLOB) 
  LOB (c) STORE AS BASICFILE (DISABLE STORAGE IN ROW RETENTION NONE);
INSERT INTO t SELECT object_id, object_name FROM all_objects WHERE rownum <= 1000;
1000 rows affected

COMMIT;
UPDATE T SET c=NULL;
1000 rows affected

COMMIT;
SELECT s.bytes/1024/1024 as mb 
  FROM user_lobs 
  JOIN user_segments s using (segment_name);

MB

9

ALTER TABLE t MODIFY LOB (c) (SHRINK SPACE);
SELECT s.bytes/1024/1024 as mb 
  FROM user_lobs 
  JOIN user_segments s using (segment_name);

MB

.0625

drop table t purge;