Geometrical solid (5)
I believe the answer is:
prism
'geometrical solid' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for prism that I've seen before include "Refracting glass" , "Triangular shape of glass or quartz, breaks up a beam" , "Light splitting optical device" , "Prims (anag)" , "White light splitter" .)