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An extra possibly removed in the theatre (8)

Ross

I believe the answer is:

appendix

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This clue could be a double definition.

'an extra possibly removed in' is the first definition.
Although both the answer and definition are singular nouns, I don't see how they can define each other.

'theatre' is the second definition.
The definition and answer can be both related to communication as well as being singular nouns.
Maybe you can see an association between them that I can't see?

'the' acts as a link.

Can you help me to learn more?

(Other definitions for appendix that I've seen before include "that may have to be removed" , "Supplementary material at the end of a document" , "Supplementary information at end of book" , "has no known use" , "Reference" .)

I've seen this clue in the Evening Standard.
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