Right to kill grass (4)
I believe the answer is:
rice
'grass' is the definition.
(thesaurus)
'right to kill' is the wordplay.
'right' becomes 'r' (abbreviation).
'to kill' becomes 'ice' (ice can informally mean to assassinate).
'r'+'ice'='RICE'
(Other definitions for rice that I've seen before include "Seeds from the paddy field" , "Cultivated food grass" , "Grain for food" , "Basmati, for example" , "Curry side dish" .)