TD Ameritrade's growth accelerator just launched an app for college debt as its first startup. Execs at the brokerage giant explain why it's a key tool to build entirely new businesses.

· TD Ameritrade recently launched the first startup out of its growth incubator, Discotech.
· Nimbly focuses on helping young people better manage and pay off their student debt.
· The service is a freemium model that offers a basic version at no cost and a premium version for $6.99 a month that includes personalized,...