Small biting fly (4)
I believe the answer is:
gnat
'small biting fly' 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 "Sucker (if female)" , "Something that bites" , "Tiny biting insect" , "Biting pest" , "Insect often hovering over stagnant water" .)