Dull books about British milk (6)
I believe the answer is:
obtuse
'dull' is the definition.
('obtuse' can be a synonym of 'dull')
'books about british milk' is the wordplay.
'books' becomes 'ot' (Old Testament).
'about' is an insertion indicator.
'british' becomes 'b' (abbreviation e.g. in 'BBC').
'milk' becomes 'use' (I've seen this in other clues).
'ot' enclosing 'b' is 'obt'.
'obt'+'use'='OBTUSE'
(Other definitions for obtuse that I've seen before include "A stupid angle?" , "Insensitive - more than a right angle" , "Dense, dim about the angle" , "Slow-witted; type of angle" , "To be thus blunt" .)