Unfairly punish small boys: one’s contrite, finally (9)
I believe the answer is:
Here is my best explanation:
'unfairly punish' is the definition.
'small boys one's contrite finally' is the wordplay.
'small boys' becomes 'victim' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'one's' becomes 'is' (Roman numeral).
'finally' indicates one should take the final letters.
The final letter of 'contrite' is 'e'.
(Other definitions for victimise that I've seen before include "Punish unjustly", "Single out unfairly" and "Bully".)