Mould; array (6)
I believe the answer is:
matrix
<span class="explanation-format__ClueExplanation">I believe this clue is a double definition.<br/ ><br/ >'<span class="explanation-format__original">mould</span>' is the first definition.<br/ ><span class="explanation-format__explanation">(I know that matrix is a type of mould)</span><br/ ><br/ >'<span class="explanation-format__original">array</span>' is the second definition.<br/ ><span class="explanation-format__explanation">(I know that matrix is a type of array)</span><br/ ><br/ ></span>
(Other definitions for matrix that I've seen before include "Grid-like pattern of elements" , "Mould; lattice" , "Casting mould" , "Rectangular array of mathematical data set out in rows and columns" , "Rectangular array of quantities or symbols" .)