Stag heard organ's essential feature (4)

I believe the answer is:

hart

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'stag' is the definition.
(both can mean a male deer)

I don't understand how the remainder of the clue works.

(Other definitions for hart that I've seen before include "Male red-deer" , "Songwriting partnership, Rodgers and --" , "providing material for Richard Rodgers?" , "Stag" , "Animal" .)

I've seen this clue in The Globe and Mail.

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