Flying pest (4)
I believe the answer is:
gnat
'flying pest' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for gnat that I've seen before include "Stinging insect" , "a little bit of a nuisance" , "Something that bites" , "Small bloodsucker" , "Tiny biting insect" .)