Shade missing from Harris Tweed - that's one for the books (6)

I believe the answer is:

writer

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'shade' is the definition.
Although both the answer and definition are singular nouns, I don't see how one could define the other.

I cannot understand how the rest of the clue works.

Can you help me to learn more?

(Other definitions for writer that I've seen before include "Scribe, author" , "Memorandum could one produce?" , "Novelist" , "Quiller" , "Author; correspondent" .)

I've seen this clue in the Irish Times (Crosaire) and the Irish Times.

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