Not the case guard used regularly to keep books in (6)
I believe the answer is:
untrue
'not the case' is the definition.
'guard used regularly to keep books in' is the wordplay.
'regularly' means one should take alternating letters (regularly take one letter, leave next etc.).
'to keep' indicates putting letters inside.
'books in' becomes 'nt' (New Testament. I am not sure about the 'in' bit.**).
'guard'+'used'='guardused'
The alternate letters of 'guardused' are 'urue'.
'urue' going around 'nt' is 'UNTRUE'.
(Other definitions for untrue that I've seen before include "Counter-factual" , "Not level" , "Not in accordance with fact" , "Not factual or honest" , "like the magnetic north?" .)