Three-sided polygon (8)
I believe the answer is:
triangle
'three-sided polygon' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for triangle that I've seen before include "It has three sides" , "Integral part of common geometric figure" , "involved in eternal emotional conflict" , "Three-sided figure - musical instrument" , "Relating to percussion instrument" .)