This accent mark: ~ (5)
I believe the answer is:
tilde
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">this accent mark</span>' is the definition.<br/ ><span class="explanation-format__explanation">(type of accent used in Spanish and Portuguese)</span><br/ ><br/ >This is all the clue.<br/ ></span>
(Other definitions for tilde that I've seen before include "Spanish cedilla" , "Squiggly mark" , "Accent (used over n) in Spanish and Portuguese" , "Style detail" , "Accent on some Spanish words" .)