Second-choice book? (7)
I believe the answer is:
reserve
'book?' is the definition.
(as in booking or reserving in advance)
'second-choice' is the wordplay.
'second' becomes 're' (prefix meaning second in eg review, re-education).
'choice' becomes 'serve' (I am not sure about this - if you are sure you should believe this answer much more).
're'+'serve'='RESERVE'
Can you help me to learn more?
(Other definitions for reserve that I've seen before include "Twelfth man < 's coolness" , "Save" , "Bank" , "Husband - substitute" , "Savings - book" .)