Eight-sided polygon (7)
I believe the answer is:
octagon
'eight-sided polygon' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for octagon that I've seen before include "Shape of a square with the corners cut off" , "Geometric figure with eight sides" , "Eight-sided figure" , "with eight angles" , "Eight-sided room" .)