Polyhedron (5)
I believe the answer is:
prism
'polyhedron' is the definition.
(prism is a kind of polyhedron)
This is the entire clue.
(Other definitions for prism that I've seen before include "Triangular glass shape, breaks up light" , "Glass triangle, separates light into colours" , "something that colours views?" , "Optical device used to invert an image" , "Crystal form" .)