Victorian hiding small crystal (5)
I believe the answer is:
prism
'crystal' is the definition.
(I've seen this before)
'victorian hiding small' is the wordplay.
'victorian' becomes 'prim' (synonyms).
'hiding' indicates putting letters inside.
'small' becomes 's' (abbreviation - e.g. clothes size).
'prim' going around 's' is 'PRISM'.
(Other definitions for prism that I've seen before include "Optical device used to invert an image" , "Light splitting optical device" , "Shape used to disperse light beams" , "Refracting glass" , "It breaks light up into rainbow's colours" .)