How to get value XML blob type in PLSQL?

How to get value C in field XMLData?

As mentioned by @Wernfried Domscheit your xml doesnot look correct. See below one way to do it:

create table traptabclob(testclob clob);
/

insert into traptabclob 
values
('<?xml version="1.0" encoding="UTF-8"?>
<DCResponse>
 <id>
   <Field key="a">Test</Field>
   <Field key="b">Test2</Field>
   <Field key="c">Test3</Field> 
  </id>                            
</DCResponse>');
/

Query:

SELECT 
EXTRACTVALUE(xmltype(testclob), '/DCResponse/id/Field[@key="c"]') col1
FROM traptabclob;

Output:

Col1
----
Test3

DEMO

Edit:

Thank you for your reply, my data doesn't has tag <?xml> version="1.0".... my data just like <row> id="1"><a>Test</a><b>Test2</b></row>, it is looks like xml type but i dont know this is xml type or not. Thank you

Please see that i have just given an example of valid xml file which contains usually <?xml> version="1.0"..... However issue with your xml code is putting tags in ". See below how it works:

create table traptabclob(testclob clob);
insert into traptabclob values('
 <id>
   <a>Test</a>
   <b>Test2</b>
   <c>Test3</c> 
  </id>');

Query:

SELECT EXTRACTVALUE(xmltype(testclob), '/id/c') col1 
FROM traptabclob ;

Output:

Col1
----
Test3

DEMO 1