One helps to observe sulphur being removed from resinous wine (6)
I believe the answer is:
retina
'one' is the definition.
Although both the answer and definition are singular nouns, I don't understand how one could define the other.
I don't understand how the rest of the clue works.
Can you help me to learn more?
(Other definitions for retina that I've seen before include "Eye's membrane" , "It helps one to see" , "Bad tear in part of the eye" , "Tear in (anag) - part of the eye" , "Light-sensitive eyeball tissue" .)