Optical toy (12)
I believe the answer is:
kaleidoscope
'optical toy' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for kaleidoscope that I've seen before include "colourful viewer" , "Diverse variety of sights" , "Toy making coloured patterns" , "A changing pattern of colours, shapes" , "Optical pattern-making toy" .)