Book a possible substitute (7)
I believe the answer is:
reserve
This clue might be a double definition.
'book a possible' is the first definition.
'reserve' can be an answer for 'book' (as in booking or reserving in advance). I am not sure about the 'a possible' bit.
'substitute' is the second definition.
(I know that substitute can be written as reserve)
Can you help me to learn more?
(Other definitions for reserve that I've seen before include "Order" , "Lack of openness" , "Kept back stock" , "Save" , "not in the front line" .)