Most modern members of flock probing religious books (6)
I believe the answer is:
newest
'most modern' is the definition.
('new' is associated in meaning with 'modern')
'members of flock probing religious books' is the wordplay.
'members of flock' becomes 'ewes' (female sheep).
'probing' indicates putting letters inside (probe can mean to poke or prod into).
'religious books' becomes 'NT' (New Testament).
'ewes' placed within 'nt' is 'NEWEST'.
(Other definitions for newest that I've seen before include "Most original" , "Superlatively modern" , "most up-to-date" , "most recently released?" , "Most recent or least worn" .)