Behind this door, books are essential (8)
I believe the answer is:
inherent
'essential' is the definition.
(I've seen this before)
'behind this door books' is the wordplay.
'behind' means one lot of letters go next to another.
'this door' becomes 'inhere' (I am not sure about this - if you are sure you should believe this answer much more).
'books' becomes 'nt' (New Testament).
'inhere'+'nt' is 'INHERENT'.
'are' acts as a link.
Can you help me to learn more?
(Other definitions for inherent that I've seen before include "Integral, innate" , "Existing permanently in something" , "Built-in, integral" , "Innate, natural" , "Necessary as a part" .)