Discussed peak cause of crime (4,7)
I believe the answer is:
high treason
'crime' is the definition.
(high treason is a kind of crime)
'discussed peak cause' is the wordplay.
'discussed peak' becomes 'hight' (I can't justify this - if you can you should believe this answer much more).
'cause' becomes 'reason' (I've seen this before).
'hight'+'reason'='HIGH TREASON'
'of' is the link.
Can you help me to learn more?
(Other definitions for high treason that I've seen before include "Capital offence" , "crime against the state?" , "Betrayal of one's country" .)