Length times width, for a rectangle (4)
I believe the answer is:
area
'length times width for a rectangle' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for area that I've seen before include "District; locale" , "Extent of a two-dimensional surface" , "field of study" , "The measure of a surface" , "Part of a building, city etc" .)