Gnat, for one (4)
I believe the answer is:
pest
'gnat for one' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for pest that I've seen before include "Destructive insect or other animal" , "Annoying thing" , "Troublesome person" , "Annoying nuisance" , "Constant nuisance" .)