First murderer to speak of old school punishment (4)
I believe the answer is:
cane
'old school punishment' is the definition.
(I've seen this before)
'first murderer to speak' is the wordplay.
'first murderer' becomes 'Cain' (Cain killed Abel in the Bible).
'to speak' indicates a 'sounds like' (homophone) clue.
'cain' is a homophone of 'CANE'.
'of' is the link.
(Other definitions for cane that I've seen before include "Walking stick; punish" , "Rod for beating" , "Stick; thrash" , "Punish; woody stem" , "Strong, flexible stem" .)