Punishment for a murderer, we hear (4)
I believe the answer is:
cane
'punishment' is the definition.
(I've seen this before)
'murderer we hear' is the wordplay.
'murderer' becomes 'cain' (Biblical figure).
'we hear' shows a homophone (sound like).
'cain' is a homophone of 'CANE'.
'for a' is the link.
(Other definitions for cane that I've seen before include "A slender rod, perhaps for punishment" , "Rod for beating" , "Kind of sugar?" , "Malacca, perhaps" , "Bamboo, sugar, stem" .)