Religious building always full of books, European (5)
I believe the answer is:
abbey
'religious building' is the definition.
(I've seen this before)
'always full of books european' is the wordplay.
'always' becomes 'ay' (archaic term for 'always').
'full of' means one lot of letters goes inside another.
'books' becomes 'BB' (an abbreviation for book written twice).
'european' becomes 'e' (abbreviation e.g. EU).
'bb'+'e'='bbe'
'ay' placed around 'bbe' is 'ABBEY'.
(Other definitions for abbey that I've seen before include "Religious building" , "Large church" , "Monks' abode" , "Religious house" , "sacred place" .)