Dismissal (4)
I believe the answer is:
'dismissal' is the definition.
(I know that dismissal can be written as sack)
This is all the clue.
(Other definitions for sack that I've seen before include "Fire (sl.)" , "destroy > 50s dress" , "Dismissal - sherry!" , "A bag for Falstaff's drink" , "Dismiss; an old wine" .)