Mould; array (6)
I believe the answer is:
matrix
I believe this clue is a double definition.
'mould' is the first definition.
(I know that matrix is a type of mould)
'array' is the second definition.
(I know that matrix is a type of array)
(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" .)