Subject of ridicule taken in by sincere comeback (8)
I believe the answer is:
rebuttal
'comeback' is the definition.
(I've seen this in another clue)
'subject of ridicule taken in by sincere' is the wordplay.
'subject of ridicule' becomes 'butt' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'taken in by' means one lot of letters goes inside another.
'sincere' becomes 'real' (similar in meaning).
'butt' going into 'real' is 'REBUTTAL'.
Can you help me to learn more?
(Other definitions for rebuttal that I've seen before include "contrary argument" , "Denial" , "Refutation, contradiction" , "Counter-statement" , "Negation" .)