Impress greatly (3)
I believe the answer is:
wow
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">impress greatly</span>' is the definition.<br/ ><span class="explanation-format__explanation">(synonyms)</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" , "Impress hugely" , "that's incredible!" , "Palindromic exclamation of surprise or admiration" , "Exclamation of admiration - another palindrome" .)