Poison in canned beef? (5)
I believe the answer is:
toxin
'poison' is the definition.
(toxin is a kind of poison)
'in canned beef' is the wordplay.
'in' is an insertion indicator.
'canned' becomes 'tin' (I can't explain this - if you can you should give a lot more credence to this answer).
'beef' becomes 'ox'.
'tin' placed around 'ox' 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" .)