Become dull (7)
I believe the answer is:
tarnish
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">become dull</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 "Spoil" , "Besmirch" , "Go dull" , "(Of metal) lose lustre" , "Take the gloss off" .)