I answered the question earlier in the thread and will repost it
What is the establishment to you claig?'
It is the rich elite who control the narrative, the ideas, the media narrative and the Overton window of policy. It is the agreed consensus of a clique of out of touch insiders who rule over millions and millions of ordinary people by spin, money, media influence and lobbyists.
Corbyn put it well, which is why they don't like him.
"The principle of democracy has to be ground up.
It is about breaking open this magical circle of Westminster, some of our great universities, Whitehall and the boardrooms who try to control thinking, control ideas and control the way policy is developed.
– Jeremy Corbyn"