Peeper part (6)
I believe the answer is:
cornea
'peeper part' is the definition.
('cornea' is part of 'peeper')
This is all the clue.
(Other definitions for cornea that I've seen before include "Clear part of the eye" , "Eyeball membrane" , "Transparent covering of the eye" , "Transparent part of the eye" , "Clear front layer of eye" .)