Cavity in the facial bones (5)
I believe the answer is:
sinus
'cavity in the facial bones' is the definition.
(I've seen this before)
This is the entire clue.
(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" .)