Source of annoyance (8)
I believe the answer is:
irritant
'source of annoyance' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for irritant that I've seen before include "Cause of itch" , "Something that annoys, a nuisance" , "distraction" , "Annoying thing" , "causing annoyance" .)