The assumption is now the potion has been mixed (8)
I believe the answer is:
adoption
'the assumption' is the definition.
(thesaurus)
'now the potion has been mixed' is the wordplay.
I cannot quite see 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' acts as a link.
Can you help me to learn more?
(Other definitions for adoption that I've seen before include "Endorsement" , "Act of taking as one's own" , "The taking on of someone else's child" , "process to select candidate" , "Acceptance (of, eg, a report)" .)