Winter transport that's murder, we hear (6)
I believe the answer is:
sleigh
'winter transport' is the definition.
(I've seen this before)
'murder we hear' is the wordplay.
'murder' becomes 'slay' (synonyms).
'we hear' shows a homophone (sound like).
'slay' is a homophone of 'SLEIGH'.
'that's' is the link.
(Other definitions for sleigh that I've seen before include "transport (with bells on!)" , "Reindeer-pulled sledge" , "Vehicle for transport over snow" , "Toboggan" , "transport in snowbound area" .)