Mathematical array (6)
I believe the answer is:
matrix
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">mathematical array</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 matrix that I've seen before include "A maths array in rows and columns" , "Grid-like pattern of elements" , "Mathematical array of numbers" , "Grid-like array of rat mix" , "Frame - template" .)