Airborne irritant (4)
I believe the answer is:
gnat
'airborne irritant' 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" , "Biting pest" , "a little bit of a nuisance" , "insect's" , "Small (swarming) fly" .)