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 present in the answer.
an anagram of 'potion' is 'option' which is located in the answer.
No letters remain.
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 "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)" .)