Small flying insect (4)
I believe the answer is:
gnat
'small flying insect' 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" , "little irritation" , "Small fly that bites" , "Tang of a biting fly" , "Small bloodsucker" .)