Brood about one's ill-treatment (6)
I believe the answer is:
misuse
'ill-treatment' is the definition.
(I've seen this before)
'brood about one's' is the wordplay.
'brood' becomes 'muse' (I can't justify this - if you can you should give a lot more credence to this answer).
'about' indicates putting letters inside.
'one's' becomes 'I's' (Roman numeral).
'muse' placed around 'is' is 'MISUSE'.
Can you help me to learn more?
(Other definitions for misuse that I've seen before include "exploitation" , "bad practice" , "Apply badly or incorrectly" , "Ill-treat" , "bad treatment" .)