Mistreated tear in part of eye (6)
I believe the answer is:
retina
'part of eye' is the definition.
(eyes have retinas)
'mistreated tear in' is the wordplay.
'mistreated' is an anagram indicator.
'tear'+'in'='tearin'
'tearin' is an anagram of 'RETINA'.
(Other definitions for retina that I've seen before include "Retain with part of eye" , "Ocular membrane" , "Eye's membrane" , "Part of the eyeball" , "Membrane covering the back of the eyeball" .)