"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.
(thesaurus)
(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)" .)