Eye part one studies (5)
I believe the answer is:
pupil
I believe this is a triple definition!
'eye' is the first definition.
(thesaurus)
'part' is the second definition.
(I have seen 'Part of eye ' mean 'pupil' so perhaps 'part' could also mean 'pupil')
'one studies' is the third definition.
(pupil can mean a student)
(Other definitions for pupil that I've seen before include "feature in "Observer"?" , "Schoolboy or eye part" , "part of my anatomy" , "Centre of iris, in the eye" , "Eye hole" .)