Of the ear or of the mouth, we hear (5)
I believe the answer is:
aural
'of the ear' is the definition.
(I've seen this before)
'of the mouth we hear' is the wordplay.
'of the mouth' becomes 'oral' (I've seen this before).
'we hear' shows a homophone (sound like).
'oral' is a homophone of 'AURAL'.
'or' acts as a link.
(Other definitions for aural that I've seen before include "To do with an ear or hearing" , "Received by the ears" , "Hearing-related" , "musical perception" , "but not in the nose!" .)