Of serfs and vassals (6)
I believe the answer is:
feudal
'of serfs and vassals' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for feudal that I've seen before include "Of times of vassals and lords" , "Of system of land-holding in Middle Ages" , "Reactionary" , "related to outdated system" , "Relating to vassalage" .)