You once repeated after Mark, "I don't believe it" (2,3)
I believe the answer is:
my eye
'i don't believe it' is the definition.
(expression of disbelief)
'you once repeated after mark' is the wordplay.
'you once' becomes 'ye' (historical version of 'you').
'repeated' indicates the doubling of some letters.
'after' means one lot of letters go next to another.
'mark' becomes 'm'.
'ye' doubled is 'yeye'.
'yeye' put after 'm' is 'MY EYE'.
(Other definitions for my eye that I've seen before include "rhyme" , "bunk?" , "Nonsense! (slang)" , "Bull" , "That's nonsense!" .)