Ponder about one's ill-treatment (6)
I believe the answer is:
misuse
'ill-treatment' is the definition.
(I've seen this before)
'ponder about one's' is the wordplay.
'ponder' becomes 'muse' ('muse' can be a synonym of 'ponder').
'about' is an insertion indicator.
'one's' becomes 'is' (roman numeral).
'muse' going around 'is' is 'MISUSE'.
(Other definitions for misuse that I've seen before include "Apply badly or incorrectly" , "Ill-treat" , "Mistreat, misapply" , "Treat badly, unfairly" , "Maltreat" .)