This helps one to see wine son overlooked (6)
I believe the answer is:
Here is my best explanation:
'this helps one to see' is the definition.
(I've seen this before)
'wine son overlooked' is the subsidiary indication.
'wine' becomes 'retsina' (type of Greek wine).
'son' becomes 's'.
'overlooked' indicates named letters should be taken away.
'retsina' with 's' removed is 'retina'.
(Other definitions for retina that I've seen before include "Retain with part of eye", "Part of the eye to retain", "something that helps conjure up image", "Light-sensitive eye layer", "Membrane in eye, sensitive to light", "part of the water pump", "sensitive part of eye", "Membrane covering the back of the eyeball", "Part of the eyeball" and "Light-sensitive eye lining".)