Heart to heart organ (3)
I believe the answer is:
ear
'organ' is the definition.
(ear is a kind of organ)
'heart to heart' is the wordplay.
'heart' indicates the answer is hidden within the clue (I've seen 'heart of' mean this).
'EAR' is hidden within 'heart'.
(Other definitions for ear that I've seen before include "Spike (wheat)" , "good sense of music" , "willingness to listen" , "Van Gogh missing body part" , "Place for a plug" .)