Organ of hearing (3)
I believe the answer is:
ear
'organ of hearing' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for ear that I've seen before include "Listening organ" , "Part of a cereal plant containing the seeds" , "Appreciation of sounds" , "It's used in hearing" , "Discernment musicians need" .)