Itty-bitty biter
I believe the answer is:
gnat
'itty-bitty biter' 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 "Tiny thing" , "Small biting fly" , "Tang (anag.)" , "a little bit of a nuisance" , "little irritation" .)