Nasal passage (5)
I believe the answer is:
sinus
'nasal passage' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for sinus that I've seen before include "Space in temple" , "Cavity in the facial bones" , "chamber" , "Cavity in body (connecting to nose)" , "Cavity in skull connecting with nostrils" .)