Pesky insect (4)
'pesky 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 "Small, swarming insect" , "Midge, mosquito" , "Woodland area" , "small irritant" , "Any of various small biting flies" .)
(Other definitions for flea that I've seen before include "high-jumper" , "Little jumping insect" , "Blood-sucking parasites" , "Tiny circus performer?" , "Small blood-sucking insect" .)