How so? Singapore has open and free elections. Anyone can run, it's just that the PAP, for all their flaws, do such a good job that there is no reason to vote them out. This is a reflection of the low corruption environment.
Or, lest we forget PRI, it is a sign of insanely high corruption. (Not saying it's the case in Singapore, just pointing out that a one party system doesn't necessarily point towards low corruption).
There's other possibilities (low press freedom, etc).
