Mix or agitate (4)
I believe the answer is:
stir
I believe this clue is a double definition.
'mix' is the first definition.
(I've seen this before)
'agitate' is the second definition.
(I know that agitate can be written as stir)
(Other definitions for stir that I've seen before include "Mix or agitate" , "Whisk or rouse" , "Move; wake" , "created by an agitator?" , "cooler" .)