Sack; stimulate (4)
I believe the answer is:
fire
I believe this clue is a double definition.
'sack' is the first definition.
(I know that sack can be written as fire)
'stimulate' is the second definition.
(fire or fire up can mean to stimulate)
(Other definitions for fire that I've seen before include "... or combustion" , "Trigger" , "Weaponry" , "Dismiss; bake in a kiln" , "Blaze - discharge" .)