Beef in can that's poisonous (5)
I believe the answer is:
toxin
'that's poisonous' is the definition.
(toxins are poisonous)
'beef in can' is the wordplay.
'beef' becomes 'ox'.
'in' means one lot of letters goes inside another.
'can' becomes 'tin' (both are metal containers).
'ox' put within 'tin' is 'TOXIN'.
(Other definitions for toxin that I've seen before include "Natural poison" , "Poisonous substance" , "Poison of organic origin" , "Poison, venom" , "Form of poison" .)