Pesky insect
I think it could be either:
'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" .)