Dumbfound (11)
'dumbfound' is the definition.
(thesaurus)
This is all the clue.
(Other definitions for flabbergast that I've seen before include "Strike dumb" , "Astonish" , "floor" , "Astound" , "stunning" .)
'dumbfound' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for stun that I've seen before include "Knock unconscious" , "deliver a knock-out blow" , "Concuss; amaze" , "Stagger, amaze" , "Amaze; knock senseless" .)