Hole in the head? (5)
I believe the answer is:
sinus
'hole in the head?' is the definition.
(a cavity in the bones of the face)
This is all the clue.
(Other definitions for sinus that I've seen before include "Air-filled cavity in one's head" , "Head space" , "Hollow space" , "Nasal cavity" , "Cavity within a bone" .)