Substitute book (7)

I believe the answer is:

reserve

This clue is a double definition.

'substitute' is the first definition.
('reserve' can be a synonym of 'substitute')

'book' is the second definition.
(I've seen this before)

(Other definitions for reserve that I've seen before include "Lack of openness" , "Secure" , "Keep back - book" , "Hold back" , "Set aside - shyness" .)

I've seen this clue in The Times.

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