Mistreat a special collection of bones (9)
I believe the answer is:
metatarsi
'collection of bones' is the definition.
(the metatarsi are bones of the foot)
'mistreat a special' is the wordplay.
'special' is an anagram indicator.
'mistreat'+'a'='mistreata'
'mistreata' with letters rearranged gives 'METATARSI'.
(Other definitions for metatarsi that I've seen before include "bones" , "Parts of the feet" .)