The most common and most likely answer to this crossword clue is the 9 letter word FEUDALISM
I've seen this crossword clue in Financial Times
F
E
U
D
A
L
I
S
M
More clues leading to the result FEUDALISM
Buddy explains!
The straight part
We know that "medieval social system" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case medieval social system = feudalism.The cryptic/wordplay part
In cryptic crossword there's often an wordplay/cryptic part as well.This part involves a more complex hint, such as an anagram, homophone, hidden word, etc.
Sadly, We don't have an explanation for this specific crossword clue yet.
What does FEUDALISM mean?
FEUDALISM noun- A social system based on personal ownership of resources and personal fealty between a suzerain (lord) and a vassal (subject). Defining characteristics are direct ownership of resources, personal loyalty, and a hierarchical social structure reinforced by religion.