Sheen appearing at last on Casualty? (5)
I believe the answer is:
gloss
'sheen' is the definition.
(I've seen this in another clue)
'appearing at last on casualty?' is the wordplay.
'at last' says to take the final letters.
'on' is a charade indicator (letters next to each other).
'casualty?' becomes 'loss' (loss is a kind of casualty).
The last letter of 'appearing' is 'g'.
'g'+'loss'='GLOSS'
(Other definitions for gloss that I've seen before include "Surface shine" , "Superficial appearance" , "Shiny type of paint" , "Very shiny finish" , "phrase's explanation" .)