Viewer's screen's made of metal with authentic coating (7)
I believe the answer is:
retinal
'metal with authentic coating' is the definition.
The answer and definition can be both substances as well as being singular nouns.
Maybe you can see an association between them that I don't see?
'viewer's screen's made' is the wordplay.
'viewer's screen' becomes 'retina' (screen-like part of the eye).
'made' becomes 'l' (I can't explain this - if you can you should give a lot more credence to this answer).
'retina'+'l'='RETINAL'
'of' acts as a link.
(Other definitions for retinal that I've seen before include "such an area filled with cones" , "Pertaining to the part of the eye sensitive to light" , "Pertaining to light-sensitive part of eye" , "Of the back layer of the eye" , "of area receiving light" .)