Book you save for a rainy day (7)

I believe the answer is:

reserve

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

I don't understand the remainder of the clue.

(Other definitions for reserve that I've seen before include "Bank" , "Secure" , "Keep back - book" , "Wine kept" , "Keep for someone - spare bod" .)

I've seen this clue in the Evening Standard.

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