create or replace function clob_to_xmltype( filestr in varchar2 ) return xmltype
is
begin
  
  ...
  
end;

SQL> CREATE TABLE clobtab (clobcol CLOB);

Table created.

SQL> INSERT INTO clobtab VALUES (
  2  '<?xml version="1.0"?>
  3   <EMP>
  4     <EMPNO>221</EMPNO>
  5     <ENAME>John</ENAME>
  6   </EMP>');

1 row created.

SQL>
SQL> CREATE OR REPLACE FUNCTION to_xmltype (clobcol CLOB)
  2  RETURN XMLTYPE AS
  3  BEGIN
  4     RETURN XMLType(clobcol);
  5  END;
  6  /

Function created.

SQL> show errors
No errors.
SQL>
SQL> SELECT to_xmltype(clobcol) FROM clobtab;

TO_XMLTYPE(CLOBCOL)
--------------------------------------------------------------------------------
<?xml version="1.0"?>
 <EMP>
   <EMPNO>221</EMPNO>
   <ENAME>John</ENAME>
 </EMP