Unfairly punish small boys: one’s contrite, finally (9)
I believe the answer is:
victimise
'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'.
'victim'+'is'+'e'='VICTIMISE'
(Other definitions for victimise that I've seen before include "Punish unjustly" , "Single out unfairly" , "Bully" .)