Gobsmacked (6)
'gobsmacked' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for amazed that I've seen before include "Greatly surprised or astonished" , "Astonished, stunned" , "Gobsmacked" , "was stunning" , "Greatly surprised, astounded" .)
(Other definitions for aghast that I've seen before include "Filled with fear and consternation" , "Deeply dismayed" , "Horrified, filled iwth consternation" , "Filled with consternation or horror" , "Amazed" .)