The assumption is that now the potion's been mixed (8)
I believe the answer is:
adoption
'the assumption' is the definition.
(thesaurus)
'that now the potion's been mixed' is the wordplay.
'that now' becomes 'AD' (AD, Anno Domini, as opposed to BC, Before Christ).
'been mixed' indicates an anagram.
'potion' is an anagram of 'option'.
'ad'+'option'='ADOPTION'
'is' is the link.
(Other definitions for adoption that I've seen before include "Appropriation from another source" , "Choosing (of a candidate for office)" , "how to create a family" , "Process of taking on another's child" , "Legal process of taking a child into one's family" .)