Key friend retaining four books (7)
I believe the answer is:
pivotal
'key' is the definition.
(thesaurus)
'friend retaining four books' is the wordplay.
'friend' becomes 'pal' (pal is a kind of friend**).
'retaining' means one lot of letters goes inside another.
'four' becomes 'iv' (roman numeral).
'books' becomes 'ot' (Old Testament).
'iv'+'ot'='ivot'
'pal' placed around 'ivot' is 'PIVOTAL'.
(Other definitions for pivotal that I've seen before include "Crucially important" , "Central - decisive" , "Of central importance" , "Of crucial importance" , "Key" .)