Impress hugely (3)
I believe the answer is:
wow
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">impress hugely</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this before)</span><br/ ><br/ >This is all the clue.<br/ ></span>
(Other definitions for wow that I've seen before include "Exclamation of surprise or praise - a palindrome" , "Great success" , "boy" , "Palindrome exclamation of astonishment or admiration" , "Expression of surprised admiration" .)