It separates light into the colour spectrum (5)
I believe the answer is:
prism
'it separates light into the colour spectrum' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for prism that I've seen before include "Light transmitter" , "Glass triangle, separates light into colours" , "Female character in The Importance of Being Earnest - optical device" , "What's on Dark Side of the Moon" , "Triangular piece of glass, optical device" .)