In episode 7 of season 2 of Revenge, Mason refers to Amanda as 'Benedict Clarke' which I assume is some kind of cultural reference pertaining to her recent activities but I am unable to figure out what it is. Any idea?
As MeatTrademark remarks in his comment (but is reluctant to post as an answer), this might be an allusion to Benedict Arnold, who fought on the American side during the American Revolution but defected to the British. His case and his person have become somewhat of a model example for treason in modern American culture, often referenced for this purpose. Though, having only TV and movie experience with modern American culture, I for myself haven't yet "heard of people being referred to as "Benedict [last_name]" in this manner as an insult", but this isn't a far stretch at all and I'd trust MeatTrademark's more practical experience there.
This cultural reference is even more likely when factoring in the additional information from the TV show. Amanda is indeed (supposedly) a Clarke, and given that IMDb says that in this episode
Amanda is ready to betray Emily to Mason
this seems like the most probable cultural reference there.