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.
I cannot quite understand how this works, but
'now' could be 'ad' (AD, Anno Domini, as opposed to BC, Before Christ) and 'ad' is located in the answer.
an anagram of 'potion' is 'option' which is within the answer.
This accounts for all the letters.
This explanation may well be incorrect...
'is' is the link.
Can you help me to learn more?
(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" .)