Cavity, hollow (5)
I believe the answer is:
sinus
This clue is a double definition.
'cavity' is the first definition.
(sinus is a kind of cavity)
'hollow' is the second definition.
(I've seen this before)
(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" .)