Leave gobsmacked
(Other definitions for stupefy that I've seen before include "Stun" , "Dumbfound" , "Astound" , "Astonish, shock" , "Knock out" .)
'leave gobsmacked' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for stun that I've seen before include "Daze with nuts" , "Daze, knock senseless" , "Put out for the count" , "Surprise greatly, knock someone''s socks off" , "Knock unconscious" .)