Surface coating that comes with age (6)
I believe the answer is:
patina
'surface coating that comes with age' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for patina that I've seen before include "well-worn surface" , "Surface shine" , "Film on copper or bronze" , "Developing surface film; sheen" , "Green coating on bronze" .)