Some volume of books teaching religion (5)
I believe the answer is:
litre
'some volume' is the definition.
(I've seen this before)
'books teaching religion' is the wordplay.
'books' becomes 'lit' (abbreviation for literature).
'teaching religion' becomes 'RE' (Religious Education).
'lit'+'re'='LITRE'
'of' acts as a link.
(Other definitions for litre that I've seen before include "French volume" , "Unit of capacity in metric system" , "Amount of liquid" , "less than two pints" , "Mcc?" .)