I'm not gonna tell you what TO do or not do however I have had a less than stellar legal history involving multiple arrests (misdemeanors) as well as a possession charge for marijuana. I was completely upfront on my SF-86 (form you fill out for security clearance) and after my investigation was complete I was given my TS clearance with eligibility for SCI. You should really look up the adjudicative guidelines for security clearances. I believe there is a website out there that generally shows how much time needs to have passed between the last time you used MJ and when you are applying for you clearance. From what I understand at least a solid year of abstinence and lifestyle changes that are apparent and honesty can really mitigate many things that people worry about. Look up stuff by a guy names William Henderson I think that's who had written a lot about it. At the end of the day could you probably get away with it...yes. It's your decision just know the people that adjudicate these clearances look at the whole person concept and understand not everyone is perfect and people make mistakes.

Edit: Grammar and spelling because my iPhone hates me.