Irritating insect (4)
I believe the answer is:
gnat
'irritating insect' 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" , "Small bloodsucker" , "small irritant" , "Small, swarming insect" , "Tiny thing" .)