A champagne with American conspirator (6)
I believe the answer is:
brutus
'conspirator' is the definition.
(I've seen this before)
'a champagne with american' is the wordplay.
'a champagne' becomes 'brut' (I have seen 'Dry champagne ' mean 'brut' so perhaps 'champagne' could also mean 'brut').
'with' says to put letters next to each other.
'american' becomes 'US' (United States).
'brut'+'us'='BRUTUS'
(Other definitions for brutus that I've seen before include "Conspirator in the murder of Julius Caesar" , "He was last to stab Julius Caesar" , "An honourable man, said Antony" , "associate of 24?" , "Murderer of Caesar" .)