Sentry left stony broke by reserve (4)

I believe the answer is:

book

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

I don't understand the remainder of the clue.

(Other definitions for book that I've seen before include "thing that shouldn't be cooked?" , "Reserve - publication" , "Reserve (tickets)" , "Reserve; tome" , "Leaves, bound" .)

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

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