Deporting criminals is no longer the done thing (11)
I believe the answer is:
extradition
'thing' is the definition.
The definition and answer can be both acts as well as being singular nouns.
Perhaps they are linked in a way I don't understand?
I cannot understand how the rest of the clue works.
Can you help me to learn more?
(Other definitions for extradition that I've seen before include "delivery to another country" , "Process of handing person over to foreign justice" , "The handing over of a criminal to another country" , "Handing over of someone to a foreign government" , "delivery from abroad" .)