Let's give it a watch, shall we?

From an IMDB review:

"Undercover Man" is a film obviously inspired by the arrest and conviction of the notorious Al Capone. In real life, Capone was a horrid mob boss and was responsible for all sorts of atrocities. But despite this, the government had a hard time pinning anything on him...mostly because witnesses knew they'd die if they testified against him. So, unable to convict him for the crimes they knew he'd committed, they investigated his finances and were able to prove he was making a fortune and not paying his taxes on these illegal gains. As a result, he was finally sentenced to prison and spent most of the rest of his life there.

In "Undercover Man", G-man Frank Warren (Glenn Ford), is frustrated that time and time again, mobsters walk because of witness intimidation. So, in frustration, they do what the Treasury Department did with Capone...they followed the money and tried building cases against the mob for failing to pay taxes. To make this more difficult, the mob's lawyer does everything he can to subvert justice...and he even tries to hire Warren in order to stop the investigation.