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