Row of columns (9)
I believe the answer is:
colonnade
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">row of columns</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this before)</span><br/ ><br/ >This is the entire clue.<br/ ></span>
(Other definitions for colonnade that I've seen before include "Arcade" , "Row of evenly spaced pillars" , "supporters in a row" , "Collection of columns" , "Row of columns supporting a roof" .)