Conspirator against Caesar not socially acceptable or cordial (6)
I believe the answer is:
cassis
'cordial' is the definition.
(type of cordial drink)
'conspirator against caesar not socially acceptable' is the wordplay.
'conspirator against caesar' becomes 'cassius' (I've seen this before).
'not' indicates named letters should be taken away.
'socially acceptable' becomes 'U' (as in U and non-U language).
'cassius' with 'u' taken out is 'CASSIS'.
'or' acts as a link.
(Other definitions for cassis that I've seen before include "French blackcurrant mixer" , "Liqueur made with blackcurrants" , "Syrupy blackcurrant liqueur" , "Blackcurrent liqueur" , "Blackcurrant cordial" .)