Protest by class, a convincing demonstration (6,6)
I believe the answer is:
object lesson
'a convincing demonstration' is the definition.
'object lesson' can be an answer for 'demonstration' (thesaurus). I am not sure about the 'a convincing' bit.
'protest by class' is the wordplay.
'protest' becomes 'object' (as in objecting to something).
'by' means one lot of letters go next to another.
'class' becomes 'lesson' (I've seen this before).
'object'+'lesson'='OBJECT LESSON'
Can you help me to learn more?
(Other definitions for object lesson that I've seen before include "punishment intended as a warning to others?" , "Instructive experience" , "shows what one should do" , "Example typifying a principle" , "What may be striking" .)