Lose lustre (7)
I believe the answer is:
tarnish
'lose lustre' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for tarnish that I've seen before include "Stain or discolour, say metal" , "Stain or blemish as with silverware" , "Cause to lose lustre, silver or reputation" , "Take the gloss off" , "Film" .)