Crossword Genius

Kept back popular book (2,7)

Ross

I believe the answer is:

in reserve

'kept back' is the definition.
(I've seen this before)

'popular book' is the wordplay.
'popular' becomes 'in'.
'book' becomes 'reserve' (synonyms).
'in'+'reserve'='IN RESERVE'

(Other definitions for in reserve that I've seen before include "available for use if needed" , "extra" , "Held back" , "kept for emergency use" , "kept for later" .)

I've seen this clue in The Guardian.
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