Four books kept by friend crucially important (7)
I believe the answer is:
pivotal
'crucially important' is the definition.
(I've seen this before)
'four books kept by friend' is the wordplay.
'four' becomes 'iv' ('IV' can be a synonym of 'four').
'books' becomes 'ot' (Old Testament).
'kept by' is an insertion indicator.
'friend' becomes 'pal' (pal is a kind of friend**).
'iv'+'ot'='ivot'
'ivot' going into 'pal' is 'PIVOTAL'.
(Other definitions for pivotal that I've seen before include "Of crucial importance" , "Of central importance" , "Crucially important" , "Crucial, critical" , "Key" .)