Good. It is probably the best thing Trump has ever done. Brennan is one of the most despicable former U.S. officials alive. He should rot in hell instead of making money off his former status.
Besides that there is no sound reason why anyone who does not work for the government, directly or indirectly, should have a clearance and thereby access to state secrets. ACLU and others are wrong in this. Revoking or keeping a security clearance has nothing to do with free speech or first amendment rights.
Abu Jihad Brennan was the CIA's station chief in Saudi Arabia. Some assert he went native, i.e. converted to Wahhabism. Brennan was in Riyadh when the Khobar Towers were bombed. Al-Qaeda did it, but Brennan was helpful in blaming the attack on Hizbullah and Iran. He was deputy executive director of the CIA on 9/11. That 9/11 happened was an intelligence failure or, as some have it, an incident arranged by the deep state.