Pi times radius squared, for a circle (4)
I believe the answer is:
area
'pi times radius squared for a circle' 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 of activity or surface" , "Space occupied by something" , "Expanse or neighbourhood" , "Territory" , "Region, space" .)