Length x width, for a rectangle (4)
I believe the answer is:
area
'length x width for a rectangle' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for area that I've seen before include "Field -- expanse" , "Extent of a two-dimensional surface" , "Locality" , "'Tract, region (4)'" , "Extent of a 2-dimensional surface" .)