Unnoticed part of the ear (4)
I believe the answer is:
otic
'of the ear' is the definition.
(I've seen this before)
'unnoticed part' is the wordplay.
'part' says the answer is hidden in the clue.
'OTIC' is hidden in the letters of 'unnoticed'.
(Other definitions for otic that I've seen before include "Relating to ears" , "organ" , "about hearing" , "of concern to the listener" , "relating to area incorporating canal" .)