ORA-31011: XML parsing failed
ORA-19202: Error occurred in XML processing
LPX-00244: invalid use of less-than ('<') character (use <)
Error at line 1
31011. 00000 -  "XML parsing failed"
*Cause:    XML parser returned an error while trying to parse the document.
*Action:   Check if the document to be parsed is valid.

create table foo(id int, xml clob, valid int default null);
insert into foo(id,xml) values (1, '<?xml version="1.0"?><root><a foo="b"></a></root>');
insert into foo(id,xml) values (2, '<?xml version="1.0"?><root><a foo="<"></a></root>');
commit;

create function is_valid_xml(xml_param clob)
return int
as
  scratch xmltype;
begin
  select xmltype(xml_param) into scratch from dual;
  return 1;
exception
  when others then
    return 0;
end;
/

SQL> update foo set valid = is_valid_xml(xml) where valid is null;
2 rows updated.

SQL> select id, valid from foo;
ID | VALID
----------
 1 |     1
 2 |     0

create function my_create_xml(p_clob clob) return xmltype is
  l_retval xmltype;
begin
    select xmltype(p_clob) into l_retval from dual;
    return l_retval;
exception
when others then
  if sqlcode != -31011 then raise;
  else return null;
  end if;
end;
/