"Ready, ___, go!" (3)
I believe the answer is:
This clue is a double definition.
'ready' is the first definition.
(set can mean ready or prepared)
'go' is the second definition.
(I know that set is a more specific form of the action come down)
(Other definitions for set that I've seen before include "Major unit of tennis match" , "Put, coagulate" , "Put down tennis games" , "Solidify; group" , "Mount (precious stone)" .)