Cavity in the facial bones (5)
I believe the answer is:
sinus
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">cavity in the facial bones</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 "Hole in the head" , "Cavity in the facial bones" , "Anatomical cavity" , "Bodily cavity" , "Nasal cavity" .)