A documentação do UTL_FILE diz :
Você terá que escrever o CLOB pedaço por pedaço. Algo assim:
DECLARE
v_clob CLOB;
v_clob_length INTEGER;
pos INTEGER := 1;
buffer VARCHAR2(32767);
amount BINARY_INTEGER := 32760;
prodFeed_file utl_file.file_type;
BEGIN
prodFeed_file := UTL_FILE.FOPEN ('CSV_DIR', 'productFeedLargo.xml', 'w', 32767);
v_clob := l_xmltype.getClobVal;
v_clob_length := length(v_clob);
WHILE pos < v_clob_length LOOP
dbms_lob.read(v_clob, amount, pos, buffer);
utl_file.put(prodFeed_file , char_buffer);
utl_file.fflush(prodFeed_file);
pos := pos + amount;
END LOOP;
utl_file.fclose(prodFeed_file);
END;
/
