Type of polyhedron (7)
I believe the answer is:
pyramid
'type of polyhedron' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for pyramid that I've seen before include "Ancient Egyptian building" , "Shape with square base and four triangular sides" , "Object with polygonal base and triangular sides meeting at point" , "A 3D shape" , "Massive Egyptian memorial" .)