Treated oxen parasite causing irritation (12)
I believe the answer is:
exasperation
'irritation' is the definition.
(I know that exasperation is a type of irritation)
'treated oxen parasite' is the wordplay.
'treated' is an anagram indicator.
'oxen'+'parasite'='oxenparasite'
'oxenparasite' with letters rearranged gives 'EXASPERATION'.
'causing' acts as a link.
(Other definitions for exasperation that I've seen before include "Fury" , "Intense irritation" , "Disgruntlement" , "Annoyance" .)