Finally restore the old organ (3)
I believe the answer is:
eye
'organ' is the definition.
(eye is a kind of organ)
'finally restore the old' is the wordplay.
'finally' suggests the final letters.
'the old' becomes 'ye'.
The final letter of 'restore' is 'e'.
'e'+'ye'='EYE'
(Other definitions for eye that I've seen before include "Oculus" , "May be private" , "centre, of a sort" , "It accommodates a pupil" , "Optical organ" .)