Horizontally (6)
I believe the answer is:
across
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">horizontally</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 across that I've seen before include "written like this?" , "what follows one here" , "...this is 'Number Ten'" , "Horizontally" , "Unlike this clue" .)