Radical cleric is a propaganda term with no particular definition. It is usually used today to describe those with clerical qualifications or training in Islam who oppose American goals or interests. It is more narrowly interpreted as just those fundamentalist clerics who advocate militant Islam and generally support a militant Islamic group or teach in madrassas.
Because of the relatively loose qualifications for a cleric in Sunni Islam, this is a very inclusive term. In Shia Islam it is much easier to recognize who is a qualified cleric.
The term radical refers to a mind-set of being willing to establish norms to achieve a more fundamental reform. It says nothing about action. Martin Luther King, Jr. could reasonably be described as a "radical cleric" of Christianity.
Every variation of political Islam has a quite different role for clerics. Democratic Islam generally advises that they remain in a process advisory role, or in a judicial role. Militant Islam often puts them out in front as exhorters of a military jihad. There has been a long tradition in Islam of tarika, lay preachers (usually Sufis) teaching variant doctrines as a means of conversion and local political convergence with resistance groups.