21 Across is drunk -- socially acceptable -- cheers! (5)
I believe the answer is:
adieu
'cheers' is the definition.
(I've seen this before)
I don't understand the remainder of the clue.
Can you help me to learn more?
(Other definitions for adieu that I've seen before include "Ciao, in French" , "Vale" , "the last word in France" , "Farewell, adios" , "the last word in Paris" .)