Validate XML stored in a CLOB is well formed or not.

You can use DBMS_XMLPARSER to parse an XML stored in a CLOB 

to find whether it is well formed or not.

I have posted this code in OTN also. 

SQL> set serverout on
SQL> DECLARE
  2    v_clob   CLOB := '<?xml version="1.0"?>
  3                      <ROWSET>
  4                      <IBSCOLYTD>
  5                      <ACTNOI>28004125</ACTNOI>
  6                      <MEMONOI>251942</MEMONOI>
  7                      <MEMODTEI>05-SEP-92</MEMODTEI>
  8                      <AMOUNTI>400</AMOUNTI>
  9                      <BRCDSI>513</BRCDSI>
 10                      <TYPEI>1</TYPEI>
 11                      <TRANSMONI>0</TRANSMONI>
 12                      </IBSCOLYTD>
 13                      <IBSCOLYTD>
 14                      <ACTNOI>28004125</ACTNOI>
 15                      <MEMONOI>251943</MEMONOI>
 16                      <MEMODTEI>04-OCT-92</MEMODTEI>
 17                      <AMOUNTI>400</AMOUNTI>
 18                      <BRCDSI>513</BRCDSI>
 19                      <TYPEI>1</TYPEI>
 20                      <TRANSMONI>0</TRANSMONI>
 21                      </IBSCOLYTD>
 22                      ';
 23    l_parser DBMS_XMLPARSER.Parser;
 24  BEGIN
 25    -- Create a parser.
 26    l_parser := DBMS_XMLPARSER.newParser;
 27    -- Parse the document 
 28    DBMS_XMLPARSER.parseClob(l_parser, v_clob);
 29    DBMS_XMLPARSER.freeParser(l_parser);
 30    DBMS_OUTPUT.put_line('Parsing Success');
 31  EXCEPTION
 32    WHEN OTHERS THEN
 33      DBMS_OUTPUT.put_line('Parsing failed');
 34      DBMS_XMLPARSER.freeParser(l_parser);
 35    
 36  END;
 37  / 
Parsing failed
 
PL/SQL procedure successfully completed.
 
SQL> DECLARE
  2    v_clob   CLOB := '<?xml version="1.0"?>
  3                      <ROWSET>
  4                      <IBSCOLYTD>
  5                      <ACTNOI>28004125</ACTNOI>
  6                      <MEMONOI>251942</MEMONOI>
  7                      <MEMODTEI>05-SEP-92</MEMODTEI>
  8                      <AMOUNTI>400</AMOUNTI>
  9                      <BRCDSI>513</BRCDSI>
 10                      <TYPEI>1</TYPEI>
 11                      <TRANSMONI>0</TRANSMONI>
 12                      </IBSCOLYTD>
 13                      <IBSCOLYTD>
 14                      <ACTNOI>28004125</ACTNOI>
 15                      <MEMONOI>251943</MEMONOI>
 16                      <MEMODTEI>04-OCT-92</MEMODTEI>
 17                      <AMOUNTI>400</AMOUNTI>
 18                      <BRCDSI>513</BRCDSI>
 19                      <TYPEI>1</TYPEI>
 20                      <TRANSMONI>0</TRANSMONI>
 21                      </IBSCOLYTD>
 22                      </ROWSET>      
 23                      '; --The last line added to make it ok.
 24    l_parser DBMS_XMLPARSER.Parser;
 25  BEGIN
 26    -- Create a parser.
 27    l_parser := DBMS_XMLPARSER.newParser;
 28    -- Parse the document 
 29    DBMS_XMLPARSER.parseClob(l_parser, v_clob);
 30    DBMS_XMLPARSER.freeParser(l_parser);
 31    DBMS_OUTPUT.put_line('Parsing Success');
 32  EXCEPTION
 33    WHEN OTHERS THEN
 34      DBMS_OUTPUT.put_line('Parsing failed');
 35      DBMS_XMLPARSER.freeParser(l_parser);
 36    
 37  END;
 38  / 
Parsing Success
 
PL/SQL procedure successfully completed.
 
SQL>