Arrangement of rows and columns (6)
I believe the answer is:
matrix
'arrangement of rows and columns' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for matrix that I've seen before include "Mould; lattice" , "Grid-like array of elements" , "Array of numbers in rows and columns" , "network of circuit elements" , "Mathematical array; The --, sci-fi film" .)