"major duty" is a correct and usable phrase in written English.
It is an informal way of referring to a task or responsibility that is particularly important or difficult. For example: "My major duty is to ensure that the event runs smoothly."
The major duty of the village's 15 police officers is directing traffic during West Point events.
A list of the major duty units can be found at http://www.defenselink.mil/news/Oct2001/d20011019ngr.pdf.mil/news/Oct2001/d20011019ngr.pdf
The other major duty of a deputy is to monitor the government, she said, something that most don't do.
The youths' major duty, they say, has been to smother support for the Movement for Democratic Change, the rival of Mr. Mugabe's ZANU-PF party, by terrorizing its leaders and their supporters, especially during elections.
General Pace, who was not nominated for a second stint as chairman, is scheduled to retire at the end of September; presiding as the president's senior military adviser over the next month's review is likely to be his final major duty.
Homer had been heard to boast that he was on familiar terms with the king of Sweden, whose major duty seemed to be handing out the Nobel medals.
One of its major duties is burn harmful, unneeded gases and light liquids into the atmosphere.
And his decision to remove all Carneiro's major duties will only make matters worse.
EXECUTIVES often say that one of their major duties is putting out fires.
It is one of the major duties that a Sufi must perform throughout his mystical journey toward union with God.
To counter this, one of Mr. Thorp's major duties is using duct tape to secure the plates, silverware and, most critically, the water glasses.
