Nasal cavity (5)
I believe the answer is:
sinus
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">nasal cavity</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 sinus that I've seen before include "Air-filled cavity in one's head" , "Air cavity in the cranial bones" , "Cavity within a bone" , "Air-filled cavity of the skull" , "Cavity linked to nose" .)