The assumption is there is now a choice (8)

I believe the answer is:

adoption

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'the assumption' is the definition.
(thesaurus)

'now a choice' is the wordplay.
'now' becomes 'AD' (AD, Anno Domini, as opposed to BC, Before Christ).
'a choice' becomes 'option' ('option' can be a synonym of 'choice').
'ad'+'option'='ADOPTION'

'is there is' is the link.
I am not very happy about this link. Some or all of it may be part of another bit of the clue.

Can you help me to learn more?

(Other definitions for adoption that I've seen before include "Process of taking on another's child" , "Acceptance (of, eg, a report)" , "takeover" , "Appropriation from another source" , "Taking-on, acceptance" .)

I've seen this clue in the Evening Standard.

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