Astonish, stupefy (4)
I believe the answer is:
stun
<span class="explanation-format__ClueExplanation">This clue is a double definition.<br/ ><br/ >'<span class="explanation-format__original">astonish</span>' is the first definition.<br/ ><span class="explanation-format__explanation">(I've seen this before)</span><br/ ><br/ >'<span class="explanation-format__original">stupefy</span>' is the second definition.<br/ ><span class="explanation-format__explanation">(synonyms)</span><br/ ><br/ ></span>
(Other definitions for stun that I've seen before include "Daze, knock senseless" , "Daze, paralyse" , "Bowl over" , "'Shock, daze (4)'" , "Shock or astonish" .)