Vacancy (9)
(Other definitions for emptiness that I've seen before include "Lack of content" , "vacuum" , "Void; hunger" , "Void state" , "Lack of substance" .)
'vacancy' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for opening that I've seen before include "Job opportunity" , "Window of opportunity" , "Rent" , "Inauguration" , "break" .)