Something studied under obligation (7)
I believe the answer is:
subject
I believe this clue is a double definition.
'something studied' is the first definition.
(eg a school subject)
'under obligation' is the second definition.
(eg subject to certain rules)
(Other definitions for subject that I've seen before include "Topic being discussed" , "Division of curriculum" , "Theme - one under rule" , "topic of conversation?" , "One owing allegiance to monarch" .)