Arsenic, poisoner's top killer (3)
I believe the answer is:
asp
'killer' is the definition.
(Kind of poisonous snake)
'arsenic poisoner's top' is the wordplay.
'arsenic' becomes 'as' (As is the chemical symbol for arsenic).
'top' suggests taking the first letters.
The first letter of 'poisoners' is 'p'.
'as'+'p'='ASP'
(Other definitions for asp that I've seen before include "Cleopatra's supposed nemesis" , "something at the zoo" , "Cleopatra-killing snake" , "Slithery thing" , "Predatory fish" .)