SQL> create table t (col clob);

Table created.

SQL> create index t_i on t(col);
create index t_i on t(col)
                      *
ERROR at line 1:
ORA-02327: cannot create index on expression with datatype LOB

SCOTT@orcl_11g> create table t (col clob)
  2  /

Table created.

SCOTT@orcl_11g> insert into t values
  2  ('Yes, you can create a context index on a clob column and
  3  use that index for Oracle Text searches.')
  4  /

1 row created.

SCOTT@orcl_11g> create index t_i on t(col) indextype is ctxsys.context
  2  /

Index created.

SCOTT@orcl_11g> select * from t where contains (col, 'Oracle') > 0
  2  /

COL
--------------------------------------------------------------------------------
Yes, you can create a context index on a clob column and
use that index for Oracle Text searches.


SCOTT@orcl_11g> 

SQL> create index t_i on t(col) indextype is ctxsys.context
  2  /
create index t_i on t(col) indextype is ctxsys.context
                                               *
ERROR at line 1:
ORA-29833: indextype does not exist