They owe allegiance to a monarch (8)
I believe the answer is:
subjects
'they owe allegiance to a monarch' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for subjects that I've seen before include "Themes, topics" , "Cubs jest about themes or subordinates" , "Areas for study or discussions" , "Things being discussed or people under rule" , "Monarchs have these" .)