Annoying one (4)
I believe the answer is:
pest
'annoying 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 "Annoying nuisance" , "Annoying thing" , "A nuisance - step on it" , "Destructive creature" , "Constant nuisance" .)