Don't be specific about any "bit" but talk around it in general terms and/or talk about how it makes you feel (the fear of disclosure, how hard it is keeping a secret, etc.) or why you "did" it and how it has changed your life, etc., just don't talk about the thing itself in any way, shape or form.

Perhaps if you told us the assignment (not what you did/know that you are afraid to respond with) we could come up with a better idea of how to avoid trouble.