Tinned beef in this form will poison you (5)
I believe the answer is:
toxin
'this form will poison you' is the definition.
Although both the answer and definition are singular nouns, I cannot understand how they can define each other.
'tinned beef in' is the wordplay.
'tinned' becomes 't' (this might be a standard abbreviation which I don't know about).
'beef' becomes 'ox'.
't'+'ox'+'in'='TOXIN'
Can you help me to learn more?
(Other definitions for toxin that I've seen before include "Poison, venom" , "Poison of organic origin" , "Poisonous substance" , "Natural poison" , "Form of poison" .)