Dark mark or stain (4)
I believe the answer is:
blot
<span class="explanation-format__ClueExplanation">I believe this clue is a double definition.<br/ ><br/ >'<span class="explanation-format__original">dark mark</span>' is the first definition.<br/ ><span class="explanation-format__explanation">(I've seen this in another clue)</span><br/ ><br/ >'<span class="explanation-format__original">stain</span>' is the second definition.<br/ ><span class="explanation-format__explanation">(synonyms)</span><br/ ><br/ ></span>
(Other definitions for blot that I've seen before include "Ink stain; disgrace" , "Dark mark/stain" , "Blemish made with ink" , "Don't do it to your copybook" , "stain on reputation" .)