Crossword Genius

Yellow-card reserve (4)

Ross

I believe the answer is:

book

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'reserve' is the definition.
(booking is a kind of reserving)

I don't understand how the remainder of the clue works.

Can you help me to learn more?

(Other definitions for book that I've seen before include "an item from the library" , "Reserve - volume" , "Bound volume" , "Written work; reserve" , "Reading volume" .)

I've seen this clue in The Telegraph.
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