Diminish, decrease (6)
I believe the answer is:
lessen
<span class="explanation-format__ClueExplanation">This clue is a double definition.<br/ ><br/ >'<span class="explanation-format__original">diminish</span>' is the first definition.<br/ ><span class="explanation-format__explanation">(synonyms)</span><br/ ><br/ >'<span class="explanation-format__original">decrease</span>' is the second definition.<br/ ><span class="explanation-format__explanation">(synonyms)</span><br/ ><br/ ></span>
(Other definitions for lessen that I've seen before include "Make smaller for lenses" , "Cut back or diminish" , "Decrease, diminish" , "Reduce in number" , "Make fewer in number" .)