In the name of Allaah, the Most Merciful, the Ever Merciful.
What is “clear disbelief”?
As-Salaamu ‘Alaykum

All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad, sallallaahu 'alyhi wa sallam, is His slave and Messenger.

This phrase was mentioned in a Hadeeth narrated on the authority of ‘Ubaadah ibn As-Saamit who said, “The Prophet called us and we gave him the pledge of allegiance for Islam, and among the conditions on which he took the pledge from us was that we were to listen and obey (orders) both at the time when we were active and at the time when we were exhausted, and during our difficult times and during our times of ease and to be obedient to the ruler and give him his due right, and not to fight against him unless we noticed him having clear disbelief for which we would have proof with us from Allaah.” [Al-Bukhari and Muslim]

Clear disbelief is the apparent and easily recognized form of disbelief. Imaam Ibn Hajar said in Fat’h Al-Baari that Al-Khattaabi said that the meaning of the word “clear” in this context means apparent and recognizable.

Allaah Knows best.