Good. It is one of the best things Trump has ever done. Brennan is one of the most despicable former U.S. officials alive. He should rot in hell.
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 is 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 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. Brennan was CIA chief of staff while the agency concocted false stories about Iraqi WMD. He was within the command line that ran the CIA torture program. It was Brennan who conspired with the Gulf dictators to hire Jihadis to destroy Libya and to attempt the same in Syria. In short - the man was always ruthless, incompetent and dishonest.