Done like poison (4)
I believe the answer is:
upas
'poison' is the definition.
(I've seen this before)
'done like' is the wordplay.
'done' becomes 'up' (both can mean finished or at an end).
'like' becomes 'as'.
'up'+'as'='UPAS'
(Other definitions for upas that I've seen before include "Antiar tree of Java" , "Tree from Java with poisonous sap" , "Source of poison" , "Tree yielding poisonous sap" , "A poisoner" .)