Pesky little fly (4)
I believe the answer is:
gnat
'pesky little fly' 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 "Something with bite" , "Small, swarming insect" , "mozzie" , "a little bit of a nuisance" , "Small biter" .)