It was not I who made that remark, I agree (3,4,2)
I believe the answer is:
you said it
'made that remark i agree' is the definition.
I can't tell whether this definition defines the answer.
'it was not i' is the wordplay.
I cannot quite understand how this works, but
'it' could be 'sa' (old-fashioned abbreviation for sex appeal) and 'sa' is located in the answer.
'not' could be 'o' (I've seen this in other clues) and 'o' is located in the answer.
'i' is within the answer.
This explanation may well be incorrect...
'who' acts as a link.
Can you help me to learn more?
(Another definition for you said it that I've seen is " Absolutely right!".)