You should describe the industry they are representing or anything else that would be of interest for your potential employer. The fact that they were BIG alone does not mean much. Start with an industry and than what is is that you've done
By the way - I would not recommend mentioning in the interview (off the records so to say) who they were - if your future employer values loyalty and compliance as well as trust and honesty ... well, what would they think of this behaviour? If you are coming from an institution that has a good name in its industry everyone would roughly know what type of clients you were dealing with anyway!