Make bright by rubbing (7)
I believe the answer is:
burnish
'make bright by rubbing' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for burnish that I've seen before include "Buff up" , "Shine by rubbing" , "Rub to shiny finish" , "Polish to a high shine" , "Polish by rubbing" .)