Duty was a moral or legal obligation.
Although old and weary, the War Doctor fights the Last Great Time War due to duty. (AUDIO: The Innocent)
Leef Apple Glyn Slitheen-Blathereen claimed that the Blathereen hunted the Slitheen for duty rather than profit. (TV: The Gift)
As a journalist, Cathy Salt felt that she had a duty to report facts. (TV: Boom Town)