Swarming pest
I believe the answer is:
gnat
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">swarming pest</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this in another clue)</span><br/ ><br/ >This is the entire clue.<br/ ></span>
(Other definitions for gnat that I've seen before include "Small (swarming) fly" , "a little bit of a nuisance" , "Stinging insect" , "Small bloodsucker" , "Tang of a biting fly" .)