Barman drinking Coke with added sodium (inter alia) in orgiastic rites (11)
I believe the answer is:
bacchanalia
'orgiastic rites' is the definition.
(ancient Roman festivals)
'barman drinking coke with added sodium inter alia' is the wordplay.
'barman' becomes 'bach' (Bach, a composer responsible for bars of music).
'drinking' means one lot of letters goes inside another.
'coke' becomes 'C' (synonyms).
'with added sodium' becomes 'na' (I can't justify this - if you can you should believe this answer much more).
'inter' indicates putting letters inside ('inter' means between or among in Latin).
'bach' placed around 'c' is 'bacch'.
'na' going within 'alia' is 'analia'.
'bacch'+'analia'='BACCHANALIA'
'in' acts as a link.
(Other definitions for bacchanalia that I've seen before include "Drunken revels - a China cabal (anag)" , "undisciplined congress" , "Roman festival; wild revelry" , "drunken party" , "orgies" .)