Side length squared, for a square (4)
I believe the answer is:
area
'side length squared for a square' 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 "District; plot" , "Speaking formally to (someone)" , "Length multiplied by breadth" , "'Tract, region (4)'" , "field of study" .)