My earlier security clearance has lapsed. Should I mention on my CV that I used to have it? No details, of course.
Not unless the Job asks what your Clearance is or requires one. In that case, it might be more of a Cover Letter thing.
If security is an issue for your new position, it might be helpful for them to know you have been vetted and cleared in the past. If you’re trying to sound more important or responsible, you might choose to highlight it as well, but it risks making you look either showy or perhaps overqualified, and your other qualifications should speak better than a clearance. If it’s entirely unrelated to your new position, it might make you look a bit clueless about your new employer, or that you’re applying for the wrong position.
Personally, I drop mine in back, and only for positions that will require the information. “Bragging” about clearance doesn’t instill any confidence, and the fact that it’s lapsed may lead to some unfavorable opinions on why. Perhaps you changed positions to one that didn’t require a clearance, or perhaps you were pushed there because of concerns. You get the idea.
It’s nice to know applicants are likely to pass a future clearance check, and it’s nice to know you have a certain degree of trust. It means your employer may not be wasting money to get you cleared. Otherwise it might hinder you. If it’s a company with an HR department, it’s not likely to make a difference. If it’s a smaller company and the HR department is the CEO’s assistant, or you’re applying directly to your future supervisor, you might trip yourself up. While it’s not a solid rule, supervisors tend to avoid hiring applicants that are more qualified and responsible than themselves.
If the job has or may need clearance, mention it casually on the cover page. If you’re not sure, drop in elsewhere, someplace useful for those who may need it during the hiring process. If it’s entirely unnecessary (burger flipperer comes to mind ) use the space to better promote your other assets instead.
If you used the clearance for a previous position, I would add the information to the job description you give for that position. “Held level whatever security clearance.”