So vain in organised trespass (8)
I believe the answer is:
invasion
'trespass' is the definition.
(thesaurus)
'so vain in organised' is the wordplay.
'organised' indicates an anagram.
'so'+'vain'+'in'='sovainin'
'sovainin' is an anagram of 'INVASION'.
(Other definitions for invasion that I've seen before include "Encroachment, intrusion" , "Hostile incursion" , "Occupation" , "Incursive attack" , "Unwelcome intrusion" .)