Swarming pest
I believe the answer is:
gnat
'swarming pest' is the definition.
(I've seen this in another clue)
This is the entire clue.
(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" .)