Organ of sight (3)
I believe the answer is:
eye
'organ of sight' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for eye that I've seen before include "Ireland's . . ., off the Dublin coast" , "Symmetrical spot" , "Look at, scrutinise" , "Place for pupil" , "where the pupil is seated" .)