Spare let? You'll have to book (7)
I believe the answer is:
reserve
This might be a double definition.
'spare let? you'll' is the first definition.
'reserve' can be an answer for 'spare' (I've seen this before). I am unsure of the 'let? you'll' bit.
'to book' is the second definition.
('reserve' can be a synonym of 'book')
'have' is the link.
Can you help me to learn more?
(Other definitions for reserve that I've seen before include "man on bench?" , "Conservation area" , "cool" , "One on bench" , "put to one side" .)