Stern woman in diverting books (9)
I believe the answer is:
fundament
'stern' is the definition.
('fundament' can be a synonym of 'stern')
'woman in diverting books' is the wordplay.
'woman' becomes 'dame' (dame is a kind of woman**).
'in' is an insertion indicator.
'diverting' becomes 'fun' ('fun' can be a synonym of 'diverting').
'books' becomes 'nt' (New Testament).
'fun'+'nt'='funnt'
'dame' put into 'funnt' is 'FUNDAMENT'.
(Other definitions for fundament that I've seen before include "bottom" , "seat" , "Underlying basis - foundation" .)