Son cutting proper glass object (5)
I believe the answer is:
prism
'glass object' is the definition.
(as in a glass prism which light is shone through)
'son cutting proper' is the wordplay.
'son' becomes 's' (genealogical abbreviation for son).
'cutting' indicates putting letters inside (some letters cut their way into a word).
'proper' becomes 'prim' (similar in meaning).
's' going inside 'prim' is 'PRISM'.
(Other definitions for prism that I've seen before include "Optical device used to deviate a beam" , "Light splitting optical device" , "Female character in The Importance of Being Ernest -- optical device" , "Theatrical Miss" , "Solid figure - light separator" .)