Civil service security clearance

I was hoping to pick the brains of some current/ex-civil servants about the security clearance process.

I'm currently moving forward in a civil service recruitment process. Coming from an NGO background, it's been quite interesting to see the difference with behavioural competencies etc. Anyway, the role in question requires security clearance as it deals with issues around high-consequence infections. Can anyone tell me a little bit more about what this entails -do they need a reference for all my career history and/or international travel etc?

The reason I'm curious is that over the course of my career I've worked for a number of small NGOs - two of which have now folded/merged with other organisations. In one of these cases, I only worked there for a short period and haven't stayed in touch personally with my supervisor... so I'm not sure how I would get a reference. This was about 5 years ago so still within the realm of fairly recent work history. I would hate to lose my chance at the role simply because some of my career history isn't directly verifiable.

I'm probably overthinking this, but it would be good to get insights from others.