Historian left out possible poisoner (3)
I believe the answer is:
ivy
'possible poisoner' is the definition.
'historian left out' is the wordplay.
'historian' becomes 'livy' (Roman historian).
'left' becomes 'l' (common abbreviation).
'out' suggests deleting specific letters.
'livy' with 'l' taken away is 'IVY'.
(Other definitions for ivy that I've seen before include "Clinging, parasitical plant" , "Clinging vine" , "Hedera helix" , "The ... League comprises USA's top universities" , "Old World vine" .)