Yellow-card reserve (4)
I believe the answer is:
book
'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" .)