Rabble-rouser used ill-defined church law in hearing (5,6)
I believe the answer is:
loose cannon
'rabble-rouser' is the definition.
I don't know anything about this answer so I can't tell whether it can be defined by this definition.
'ill-defined church law in hearing' is the wordplay.
'ill defined' becomes 'loose' (I can't justify this - if you can you should believe this answer much more).
'church law' becomes 'canon' (I've seen this in another clue).
'in hearing' indicates a 'sounds like' (homophone) clue.
'canon' is a homophone of 'cannon'.
'loose'+'cannon'='LOOSE CANNON'
'used' acts as a link.
Can you help me to learn more?
(Other definitions for loose cannon that I've seen before include "Unpredictable character" , "liability" , "unpredictable type" , "Uncontrolled member of a group, who may cause damage" , "I'm a threat to all" .)