Job opening (4)
'job opening' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for slot that I've seen before include "Small opening, for a coin say" , "Narrow opening as in gaming machine" , "Space for coin or programme" , "Fit into puzzle" , "Scheduled time -- aperture for coins" .)
(Other definitions for vacancy that I've seen before include "Unoccupied situation" , "Unoccupied post" , "Lack of consideration" , "Unoccupied state, job or apartment" , "An opening, job or apartment" .)