Source of irritation largely dissipated (7)
I believe the answer is:
allergy
'source of irritation' is the definition.
The definition and answer can be both states of affairs as well as being singular nouns.
Perhaps you can see an association between them that I don't see?
'largely dissipated' is the wordplay.
'dissipated' indicates anagramming the letters.
'largely' is an anagram of 'ALLERGY'.
Can you help me to learn more?
(Other definitions for allergy that I've seen before include "Immune reaction" , "Hypersensitive reaction of the body to a foreign substance" , "Largely to do with adverse reaction" , "Adverse reaction (med.)" , "Bad immune system response" .)