Pesky bug (4)
I believe the answer is:
gnat
'pesky bug' 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 "Tiny thing" , "Small (swarming) fly" , "Small biting flying insect" , "Tang (anag.)" , "Small biting fly" .)