Plane figure with a number of straight sides (7)
I believe the answer is:
polygon
'plane figure with a number of straight sides' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for polygon that I've seen before include "Geometric figure with five or more sides" , "shape" , "Plane figure with a number of straight sides" , "Straight-sided figure" , "Closed plane figure bounded by straight sides" .)