Big cat quietly sacrificed reproductive organ (6)
I believe the answer is:
anther
'reproductive organ' is the definition.
(reproductive organ in plants)
'big cat quietly sacrificed' is the wordplay.
'big cat' becomes 'panther' (panthers are big cats).
'quietly' becomes 'p' (p for piano and quiet).
'sacrificed' is a deletion indicator.
'panther' with 'p' removed is 'ANTHER'.
(Other definitions for anther that I've seen before include "Part of a plant that contains pollen" , "Pollen-holding part of stamen" , "Pollen container of the flower" , "(Actors) leave stage" , "Pollen part of flower" .)