"I am amazed" is not your line! (3,4,3)
I believe the answer is:
you dont say
'your line' is the definition.
I know nothing about this answer so I can't judge whether this works.
'i am amazed is not' is the wordplay.
I cannot quite see how this works, but
'i' could be 'a' and 'a' is found in the answer.
an anagram of 'not' is 'ont' which is located in the answer.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for you dont say that I've seen before include "No kidding!" , "Ironic response to an obvious remark" , "one thinks" , "Surprised comment" , "How astonishing,'" .)