Poison beef found in can (5)
I believe the answer is:
toxin
'poison' is the definition.
(toxin is a kind of poison)
'beef found in can' is the wordplay.
'beef' becomes 'ox'.
'found in' is an insertion indicator.
'can' becomes 'tin' ('tin' can be a synonym of 'can').
'ox' placed inside 'tin' is 'TOXIN'.
(Other definitions for toxin that I've seen before include "Poison, venom" , "Poison of organic origin" , "Poisonous substance" , "Natural poison" , "Form of poison" .)