Crossword Genius

In which to be unconscious of an optical fault? (4)

Ross

I believe the answer is:

coma

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This could be a double definition.

'in which' is the first definition.
Although both the answer and definition are singular nouns, I don't understand how they can define each other.

'unconscious of an optical fault?' is the second definition.
The answer and definition can be both related to thinking as well as being singular nouns.
Perhaps there's an association between them I don't understand?

'to be' is the link.

Can you help me to learn more?

(Other definitions for coma that I've seen before include "Unconsciousness from which person cannot be aroused" , "State of insensibility" , "Long period of unconsciousness" , "Stupor" , "State of unconsciousness caused by illness" .)

I've seen this clue in the Evening Standard.
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