Lose lustre (7)
I believe the answer is:
tarnish
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">lose lustre</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this before)</span><br/ ><br/ >This is all the clue.<br/ ></span>
(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" .)