Bodily cavity (5)
I believe the answer is:
sinus
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">bodily cavity</span>' is the definition.<br/ ><span class="explanation-format__explanation">(sinus is a kind of bodily cavity)</span><br/ ><br/ >This is all the clue.<br/ ></span>
(Other definitions for sinus that I've seen before include "Hollow space" , "Bodily cavity" , "snotty enclosure" , "chamber" , "Air cavity in the cranial bones" .)