Book a possible substitute (7)
I believe the answer is:
reserve
This clue could 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'm not sure about the 'a possible' bit.
'substitute' is the second definition.
(synonyms)
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" .)