Top secret clearance in the military is equivilent to a "Q" clearance in DOE. They usually do extensive background checks so it is possible. If you have the record of the purchase showing the itemized list of what you bought, then disclose it pointing out that it was not illegal or banned at the time.
I've never had a clearence like that, so don't take my word for it.
Make sure that you disclose any other drug use as well. I would expect that buying synthetic drugs would bring up questions of whether or not you also bought real ones.
Not John Cleese, just a much younger and thinner me.