Book store (7)

I think it could be either:

Which length description are you looking for?:

reserve

I believe this is a double definition.

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

'store' is the second definition.
(a reserve is a store or supply of something)

(Other definitions for reserve that I've seen before include "ice" , "Savings - book" , "Conservation area" , "Hold back" , "Husband - substitute" .)

I've seen this clue in The Sun, The Hindu, The Globe and Mail and The Guardian.

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