Render speechless (4)
'render speechless' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for stun that I've seen before include "Render insensitive" , "Make dazed or numb" , "cause amazement" , "Knock unconscious" , "Render temporarily senseless" .)
(Other definitions for stupefy that I've seen before include "Astonish, shock" , "Knock out" , "Astound" , "Stun" , "Dumbfound" .)