Metal casing over unknown poison (5)
I believe the answer is:
toxin
'poison' is the definition.
(I know that toxin is a type of poison)
'metal casing over unknown' is the wordplay.
'metal' becomes 'tin' (tin is a kind of metal).
'casing' means one lot of letters goes inside another (I've seen this in other clues).
'over unknown' becomes 'ox' (I can't justify this - if you can you should believe this answer much more).
'tin' enclosing 'ox' is 'TOXIN'.
(Other definitions for toxin that I've seen before include "Poison of organic origin" , "Poison, venom" , "Form of poison" , "Natural poison" , "Poisonous substance" .)