Knows the sound of an organ (4)
I believe the answer is:
nose
'an organ' is the definition.
(I know that nose is a type of Nose)
'knows the sound' is the wordplay.
'the sound' indicates a 'sounds like' (homophone) clue.
'knows' sounds like 'NOSE'.
'of' acts as a link.
(Other definitions for nose that I've seen before include "Front of plane" , "Pry" , "Facial part" , "Deserving blame" , "Faculty for detecting" .)