What a patient may lose in the theatre (13)

I believe the answer is:

consciousness

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'theatre' is the definition.
Although both the answer and definition are singular nouns, I cannot understand how one could define the other.

I can't explain the rest of the clue.

Can you help me to learn more?

(Other definitions for consciousness that I've seen before include "State of awareness" , "State of being aware" , "Self-awareness" , "Mental awareness" , "Intuition, perception" .)

I've seen this clue in The Globe and Mail.

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