It’s an editor’s rambling essay (12)
I believe the answer is:
dissertation
'essay' is the definition.
(thesaurus)
'it's an editor's rambling' is the wordplay.
'rambling' indicates an anagram.
'its'+'an'+'editors'='itsaneditors'
'itsaneditors' is an anagram of 'DISSERTATION'.
(Other definitions for dissertation that I've seen before include "writing" , "thesis" , "Essay for a degree or diploma" , "scholarly work?" .)