Cavity, opening (4)
I believe the answer is:
hole
I believe this clue is a double definition.
'cavity' is the first definition.
(hole is a kind of cavity)
'opening' is the second definition.
(I know that hole is a type of opening)
(Other definitions for hole that I've seen before include "Nasty place" , "Hollow, cavity" , "excavation" , "depression" , "pocket" .)