Optical defect of which one is unconscious? (4)
I believe the answer is:
coma
'unconscious?' is the definition.
The definition and answer can be both related to thinking as well as being singular nouns.
Maybe there's an association between them I don't understand?
I don't understand how the rest of the clue works.
Can you help me to learn more?
(Other definitions for coma that I've seen before include "State of unconsciousness caused by illness" , "Deep trance" , "Unconscious state" , "State of insensibility" , "State of deep (often prolonged) unconsciousness" .)