Something that causes annoyance (8)
I believe the answer is:
irritant
'something that causes annoyance' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for irritant that I've seen before include "Trouble" , "Source of continual annoyance" , "Something that chafes or annoys" , "distraction" , "Annoying factor" .)