Defile, sully (5)
I believe the answer is:
taint
<span class="explanation-format__ClueExplanation">This clue is a double definition.<br/ ><br/ >'<span class="explanation-format__original">defile</span>' is the first definition.<br/ ><span class="explanation-format__explanation">(synonyms)</span><br/ ><br/ >'<span class="explanation-format__original">sully</span>' is the second definition.<br/ ><span class="explanation-format__explanation">(synonyms)</span><br/ ><br/ ></span>
(Other definitions for taint that I've seen before include "Source of infection" , "Defile" , "Adulterate" , "Blemish" , "Corrupt" .)