CLOB fields do support the SQL update statement (the statement
update xyztable set clob_col='this is a modified clob col val';

will work as expected.

And you do not need to set any length attributes for CLOB fields either

With PL/SQL you can populate a CLOB col with a max value of 32767 (using the max length supported for char/varchar2 types in pl/sql). If you need to populate the CLOB col with larger values you will have to use the DBMS_LOB package functions WRITE or WRITEAPPEND.

-amar