At the Sorbonne, it’s the done thing (4,8)
I believe the answer is:
fait accompli
'thing' is the definition.
(thesaurus)
I cannot understand how the remainder of the clue works.
Can you help me to learn more?
(Other definitions for fait accompli that I've seen before include "A done deal" , "Irreversible action" , "Something irreversibly decided" , "Something done and unalterable" , "Irreversible act" .)