Poison drink? That's the limit! (6)
I believe the answer is:
margin
'the limit' is the definition.
(thesaurus)
'poison drink?' is the wordplay.
'poison' becomes 'mar' (I can't justify this - if you can you should give a lot more credence to this answer).
'drink?' becomes 'gin' (gin is a kind of drink).
'mar'+'gin'='MARGIN'
'that's' is the link.
Can you help me to learn more?
(Other definitions for margin that I've seen before include "Blank space around a text" , "Bland space around text" , "Edge or space for error" , "Blank space that surrounds the text on a page" , "bank" .)