Poison can surround a source of beef (5)
I believe the answer is:
toxin
'poison' is the definition.
(I know that toxin is a type of poison)
'can surround a source of beef' is the wordplay.
'can' becomes 'tin' (both are metal containers).
'surround' indicates putting letters inside.
'a source of beef' becomes 'ox' (I can't justify this - if you can you should believe this answer much more).
'tin' enclosing 'ox' is 'TOXIN'.
Can you help me to learn more?
(Other definitions for toxin that I've seen before include "Form of poison" , "Natural poison" , "Poison of organic origin" , "Poisonous substance" , "Poison, venom" .)