A thousand I will murder (4)
I believe the answer is:
kill
'murder' is the definition.
(I know that murder is a kind of kill)
'a thousand i will' is the wordplay.
'a thousand' becomes 'k' (K can mean thousand, e.g. 10k).
'i will' becomes 'I'll' (abbreviation).
'k'+'ill'='KILL'
(Other definitions for kill that I've seen before include "Slaughter" , "Cause death" , "Execute" , "Slay" , "Put an end to" .)