Impair, tarnish (3)
I believe the answer is:
mar
<span class="explanation-format__ClueExplanation">This clue is a double definition.<br/ ><br/ >'<span class="explanation-format__original">impair</span>' is the first definition.<br/ ><span class="explanation-format__explanation">(synonyms)</span><br/ ><br/ >'<span class="explanation-format__original">tarnish</span>' is the second definition.<br/ ><span class="explanation-format__explanation">(I've seen this in another clue)</span><br/ ><br/ ></span>
(Other definitions for mar that I've seen before include "Cause harm" , "Detract from" , "Spoil, damage" , "Spoil something" , "Ruin" .)