Nasal aperture (7)
I believe the answer is:
nostril
'nasal aperture' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for nostril that I've seen before include "Nasal aperture" , "Breathing aperture" , "Nasal cavity" , "Through which one would snort" , "Nasal passageway" .)