Monastery head’s answer books: Old Testament (5)
I believe the answer is:
abbot
'monastery head's' is the definition.
(I've seen this before)
'answer books old testament' is the wordplay.
'answer' becomes 'a' (abbreviation eg in Q and A).
'books' becomes 'BB' (an abbreviation for book written twice).
'old testament' becomes 'ot'.
'a'+'bb'+'ot'='ABBOT'
(Other definitions for abbot that I've seen before include "Head of monastery" , "Head monk" , "Monks' leader" , "Senior monk" , "Supervisor of brothers" .)