Rectangular but not square (6)
I believe the answer is:
oblong
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">rectangular but not square</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 oblong that I've seen before include "Rectangular but not square" , "Oval" , "book form?" , "Strip for planes" , "tablet format?" .)