Cavity walls in usual houses (5)
I believe the answer is:
sinus
'cavity' is the definition.
(I know that sinus is a type of cavity)
'walls in usual houses' is the wordplay.
'houses' indicates a hidden word (part of the clue houses the answer).
'SINUS' can be found hidden inside 'walls in usual'.
(Other definitions for sinus that I've seen before include "chamber" , "Airspace" , "Channel between brain and skull" , "Passage in the head" , "Air-filled cavity of the skull" .)