Taken to be the done thing (8)
I believe the answer is:
accepted
'taken' is the definition.
(accepting is a kind of taking)
'be the done thing' is the wordplay.
'be' becomes 'accept' (accepting is a kind of being).
'the done thing' becomes 'ed' (I can't justify this - if you can you should give a lot more credence to this answer).
'accept'+'ed'='ACCEPTED'
'to' acts as a link.
Can you help me to learn more?
(Other definitions for accepted that I've seen before include "Believed to be valid" , "Taken" , "Received; approved" , "Received something offered" , "Established" .)