Put books on top of ancient organ (8)
I believe the answer is:
placenta
'organ' is the definition.
(I know that placenta is a organ)
'put books on top of ancient' is the wordplay.
'put' becomes 'place' (place can mean to put something somewhere).
'books' becomes 'NT' (New Testament).
'on top of' indicates taking the first letters.
The initial letter of 'ancient' is 'a'.
'place'+'nt'+'a'='PLACENTA'
(Other definitions for placenta that I've seen before include "Organ feeding a foetus" , "Plate can appear in organ in womb during pregnancy" , "baby food" , "primary food supplier" , "temporary organ" .)