Cavity, hollow (5)
I believe the answer is:
sinus
<span class="explanation-format__ClueExplanation">This clue is a double definition.<br/ ><br/ >'<span class="explanation-format__original">cavity</span>' is the first definition.<br/ ><span class="explanation-format__explanation">(sinus is a kind of cavity)</span><br/ ><br/ >'<span class="explanation-format__original">hollow</span>' is the second definition.<br/ ><span class="explanation-format__explanation">(I've seen this before)</span><br/ ><br/ ></span>
(Other definitions for sinus that I've seen before include "Passage in the head" , "Cavity in skull connecting with nostrils" , "Air-filled cavity of the skull" , "Cavity linked to nose" , "snotty enclosure" .)