One with authority to kill legally inserting IV (9)
I believe the answer is:
executive
'one with authority' is the definition.
I can't tell whether this definition defines the answer.
'kill legally inserting iv' is the wordplay.
'kill legally' becomes 'execute' (executing is a kind of killing. I am not sure about the 'legally' bit.).
'inserting' is an insertion indicator.
'execute' going around 'iv' is 'EXECUTIVE'.
'to' acts as a link.
Can you help me to learn more?
(Other definitions for executive that I've seen before include "Big guns" , "Senior businessperson" , "Administrator, manager" , "Company director" , "Branch of government charged with putting laws, agreements etc into effect" .)