Canned beef, it seems, is not good for you (5)
I believe the answer is:
toxin
'not good for you' is the definition.
The definition suggests a singular noun which matches the answer.
'canned beef it seems' is the wordplay.
I cannot quite see how this works, but
'beef' could be 'ox' and 'ox' is located in the answer.
'it' could be 't' (abbreviation. e.g. in 'tis) and 't' is found in the answer.
The remaining letters 'in' is a valid word which might be clued in a way I don't see.
This may be the basis of the clue (or it may be nonsense).
'is' acts as a link.
Can you help me to learn more?
(Other definitions for toxin that I've seen before include "Form of poison" , "Poison, venom" , "Poison of organic origin" , "Poisonous substance" , "Natural poison" .)