This is the best authoritative source that I could find. The few assertions I've seen that Singapore is less corrupt than most european and north american countries quote Transparency International as a source, so I assume it might be an acceptable one. They also supplement their findings with a "GCB" survey[1] (which is less recent), and given enough time I guess there are other sources one could find.

These are surveys, so there's a limit to how reliable their results can be. However, I find surveys are a pretty good start when you need to get a general idea on how something does and you can't just measure it. How you value that is up to you, that's why I quote it in my comment.

[1] https://www.transparency.org/en/gcb