Rectangular (6)
I believe the answer is:
oblong
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">rectangular</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 oblong that I've seen before include "Shape of a book" , "Elongated rectangle" , "Plane figure that deviates from a square by elongation" , "Oval" , "Rectangular two-dimensional figure" .)