A British book on Old Testament religious leader (5)
I believe the answer is:
abbot
'religious leader' is the definition.
(head of an abbey)
'a british book on old testament' is the wordplay.
'british' becomes 'b' (abbreviation e.g. in 'BBC').
'book' becomes 'b' (abbreviation for book).
'on' means one lot of letters go next to another.
'old testament' becomes 'ot'.
'a'+'b'+'b'+'ot'='ABBOT'
(Other definitions for abbot that I've seen before include "head in order" , "Religious leader" , "Father" , "Prior's clerical superior" , "Head of monastery" .)