Crossword Genius

Optical defect of which one is unconscious? (4)

Ross

I believe the answer is:

coma

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'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" .)

I've seen this clue in the Evening Standard.
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