Surface gloss (5)
'surface gloss' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for sheen that I've seen before include "Radiance" , "actor?" , "Superficial brightness" , "splendid clothing" , "Acting family" .)
(Other definitions for veneer that I've seen before include "Thin layer of wood or plastic" , "Thin covering on top, as of wood" , "Capital of Kenya" , "Misleading external appearance" , "Thin covering on the outside" .)