Stag heard organ's essential feature (4)
I believe the answer is:
hart
'stag' is the definition.
(both can mean a male deer)
I don't understand how the remainder of the clue works.
Can you help me to learn more?
(Other definitions for hart that I've seen before include "Male red-deer" , "Songwriting partnership, Rodgers and --" , "providing material for Richard Rodgers?" , "Stag" , "Animal" .)