Nasal aperture (7)
I believe the answer is:
nostril
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">nasal aperture</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this before)</span><br/ ><br/ >This is all the clue.<br/ ></span>
(Other definitions for nostril that I've seen before include "Nasal aperture" , "Breathing aperture" , "Nasal cavity" , "Through which one would snort" , "Nasal passageway" .)