Organ? Yes, by the sound of it (3)
I believe the answer is:
eye
'organ?' is the definition.
(eye is a kind of organ)
'yes by the sound of it' is the wordplay.
'yes' becomes 'aye' (I've seen this before).
'by the sound of it' shows a homophone (sound like).
'aye' is a homophone of 'EYE'.
(Other definitions for eye that I've seen before include "Place for pupil" , "Watch - middle of cyclone" , "9 across village" , "Symmetrical spot" , "Four [NEEDLEFUL] may pass through this" .)