Substitute's book (7)

I believe the answer is:

reserve

I believe this clue is a double definition.

'substitute's' is the first definition.
(synonyms)

'book' is the second definition.
(as in booking or reserving in advance)

(Other definitions for reserve that I've seen before include "Conservation area" , "some distance" , "not in the front line" , "keep for oneself" , "Set aside - shyness" .)

I've seen this clue in The Daily Express.

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