Sulphur in proper crystal (5)
I believe the answer is:
prism
'crystal' is the definition.
(I've seen this before)
'sulphur in proper' is the wordplay.
'sulphur' becomes 's' ('S' can be a synonym of 'sulphur').
'in' is an insertion indicator.
'proper' becomes 'prim' ('prim' can be similar in meaning to 'proper').
's' put within 'prim' is 'PRISM'.
(Other definitions for prism that I've seen before include "Triangular piece of glass to deviate a beam" , "Transparent solid for dispersing light" , "Light splitting optical device" , "Glass triangle, separates light into colours" , "Three-sided object that separates light into colours" .)