Organ put next to altar, initially (8)
I believe the answer is:
placenta
'organ' is the definition.
(I know that placenta is a organ)
'put next to altar initially' is the wordplay.
'put' becomes 'place' (place can mean to put something somewhere).
'initially' says to take the initial letters.
The initial letters of 'next to altar' is 'nta'.
'place'+'nta'='PLACENTA'
(Other definitions for placenta that I've seen before include "Organ feeding a foetus" , "baby food" , "21 that feeds the embryo" , "Plate can appear in organ in womb during pregnancy" , "Source of nourishment for embryo" .)