I heard an organ (3)
I believe the answer is:
eye
'an organ' is the definition.
(I know that eye is a type of sensory receptor)
'i heard' is the wordplay.
'heard' indicates a 'sounds like' (homophone) clue.
'i' sounds like 'EYE'.
(Other definitions for eye that I've seen before include "Look at; centre" , "Glance" , "Sense organ" , "Ireland's . . ., off the Dublin coast" , "Oculus" .)