Clapper and Brennan are prime candidates.
Let's see:
U.S. intelligence official,
not in the FBI,
someone very well known,
very high up,
very senior in the intelligence world.
Brennan was the Director of CIA under Obama. Clapper was Director of National Intelligence. But don't forget that there were other CIA directors under Obama: Leon Panetta, Michael Morell and David Petraeus. Morell doesn't qualify as "very well known."
There were two Directors of the National Security Agency under Obama, but neither qualify as "very well known."
So using the above criteria, the leaker must be one of only four people (Brennan, Clapper, Panetta, Petraeus), possibly five if Loretta Lynch qualifies. But she was Attorney General, technically not an intelligence official.
So you're probably right. The most likely culprit is Brennan or Clapper.