Old historian’s account embodied in biblical book (7)
I believe the answer is:
tacitus
'old historian's' is the definition.
(I've seen this before)
'account embodied in biblical book' is the wordplay.
'account' becomes 'ac' (abbreviation for account).
'embodied in' is an insertion indicator.
'biblical book' becomes 'Titus' (the Epistle of Paul to Titus, a book of the Bible).
'ac' placed into 'titus' is 'TACITUS'.
(Other definitions for tacitus that I've seen before include "Old history master" , "Writer" , "Roman historian, annalist" , "Old historian" , "Roman politician and historian" .)