Put books on top of Allen organ (8)
I believe the answer is:
placenta
'organ' is the definition.
(I have seen 'temporary organ' mean 'placenta' so perhaps 'organ' could also mean 'placenta')
'put books on top of allen' is the wordplay.
'put' becomes 'place' (place can mean to put something somewhere).
'books' becomes 'NT' (abbreviation for New Testament).
'on top of' suggests taking the first letters.
The first letter of 'allen' is 'a'.
'place'+'nt'+'a'='PLACENTA'
(Other definitions for placenta that I've seen before include "21 that feeds the embryo" , "primary food supplier" , "baby food" , "Organ feeding a foetus" , "Plate can appear in organ in womb during pregnancy" .)