Try using the to_clob() function, and then you can also use all of the methods of dbms_lob. As in:

select dbms_lob.compare(to_clob(tab_a.mycolumn), to_clob(tab_b.mycolumn))
from table_a
inner join table_b
on table_a.pkey = table_b.pkey
where table_a.pkey.123456;