You didn't share much information so it is difficult to guess what you did and why Oracle complained about it. Though, would TO_NCHAR do any good?
TO_NCHAR (character) converts a character string, CHAR, VARCHAR2, CLOB, or NCLOB value to the national character set. The value returned is always NVARCHAR2
SQL> create table test (col_nclob nclob);
Table created.
SQL> create table test2 (col_nvarchar2 nvarchar2(20));
Table created.
SQL> insert into test values ('x');
1 row created.
SQL> insert into test2 (col_nvarchar2)
2 select to_nchar(t.col_nclob) from test t;
1 row created.
SQL>
You didn't share much information so it is difficult to guess what you did and why Oracle complained about it. Though, would
TO_NCHARdo any good?