Mistreat a treatment for bones (9)
I believe the answer is:
metatarsi
'for bones' is the definition.
(I've seen this before)
'mistreat a treatment' is the wordplay.
'treatment' indicates anagramming the letters (letters treated or processed differently).
'mistreat'+'a'='mistreata'
'mistreata' is an anagram of 'METATARSI'.
(Other definitions for metatarsi that I've seen before include "Parts of the feet" , "bones" .)