For a quick hack, you can create an additional column that is the lower case of the clob, and then use instr on the lower case version
create table keyword_test (
id integer primary key,
contents clob,
contents_lower clob
);
insert into keyword_test (id,contents) values (1, 'This is string ONE');
insert into keyword_test (id,contents) values (2, 'This is string TWO');
commit;
update keyword_test set contents_lower=lower(contents);
commit;
SQL> select id, dbms_lob.instr(contents_lower, 'one')
from keyword_test;
ID DBMS_LOB.INSTR(CONTENTS_LOWER,'ONE')
---------- ------------------------------------
1 16
2 0
You can also use interMedia to provide keyword searching on the clob
