Note chicken covered in spots finally healthy in Russian pen (12)

I believe the answer is:
solzhenitsyn
'pen' is the definition.
Both the answer and definition are singular nouns.
Maybe there's a link between them I don't understand?
I cannot understand how the remainder of the clue works.
Can you help me to learn more?
(Another definition for solzhenitsyn that I've seen is " A. -, Russian novelist (The Gulag Archipelago)".)
I've seen this clue in The Times and The Sunday Times.
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