Duty becomes a habit (6)
I believe the answer is:
custom
I believe this clue is a double definition.
'duty' is the first definition.
(custom is a kind of duty)
'a habit' is the second definition.
(I've seen this before)
'becomes' is the link.
(Other definitions for custom that I've seen before include "Established usage" , "'Habit, practice (6)'" , "Usual way of doing something" , "Regular trade" , "Tradition; habit" .)