clean as a whistle

「ピカピカに清潔な」という意味ですが、転じて「犯罪を犯していない／犯罪歴がない」という意味で使われます。

Time For America's CIO Vivek Kundra To Resign on Silicon Alley Insider

Even if Vivek is clean as a whistle, his effectiveness as a reformer has already been crippled.

Vivek became the first CIO of the country, and this article was written when his former subordinates got arrested.