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 subsidiary indication.
'small boys' becomes 'victim' (I am not sure about this - if you are sure you should believe this answer much more).
'one's' becomes 'is' (resembles I's).
'finally' indicates one should take the final letters.
The last letter of 'contrite' is 'e'.
(Other definitions for victimise that I've seen before include "Punish unjustly", "Single out unfairly" and "Bully".)