Agreement in a set of books written by witches (8)
I believe the answer is:
covenant
'agreement' is the definition.
(covenant is a kind of agreement)
'a set of books written by witches' is the wordplay.
'a set of books' becomes 'ant'.
'written by' says to put letters next to each other.
'witches' becomes 'coven' (I've seen this before).
'ant' after 'coven' is 'COVENANT'.
'in' is the link.
(Other definitions for covenant that I've seen before include "Concordat, compact" , "Solemn agreement" , "pledge" , "Formal agreement between two or more parties" , "Formal agreement in a convent" .)