Tiny biting insect (4)
I believe the answer is:
gnat
'tiny biting insect' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for gnat that I've seen before include "a little bit of a nuisance" , "Something that bites" , "Tiny thing" , "Sucker (if female)" , "mozzie" .)