Hole ___ (5)
'hole' is the definition.
(I know that cavity is a type of hole)
This is all the clue.
(Other definitions for cavity that I've seen before include "Destination for a filling!" , "Hollow place" , "Hole, gap" , "A hole could mean work for the dentist" , "Hollow space" .)
(Another definition for inone that I've seen is " United".)