Thick books about British operation (6)
I believe the answer is:
obtuse
'thick' is the definition.
(I've seen this before)
'books about british operation' is the wordplay.
'books' becomes 'OT' (Old Testament).
'about' is an insertion indicator.
'british' becomes 'b' (abbreviation e.g. in BBC).
'operation' becomes 'use' (I've seen this in another clue).
'ot' enclosing 'b' is 'obt'.
'obt'+'use'='OBTUSE'
(Other definitions for obtuse that I've seen before include "Dull, non-perceptive" , "Not very perceptive or sensitive" , "Blunt, dull-witted" , "Blunt - stupid - insensitive" , "Slow-witted; type of angle" .)