I will repeat: 'That's ignorant!' (10)
I believe the answer is:
illiterate
'ignorant' is the definition.
('illiterate' can be a synonym of 'ignorant')
'i will repeat' is the wordplay.
'i will' becomes 'ill' (I've seen this before).
'repeat' becomes 'iterate' ('iterate' can be a synonym of 'repeat').
'ill'+'iterate'='ILLITERATE'
'that's' is the link.
(Other definitions for illiterate that I've seen before include "Analphabetic" , "dunce" , "twenty seven?" , "lacking in education" , "Ignorant" .)