The most common and most likely answer to this crossword clue is the 6 letter word FEUDAL
I've seen this crossword clue in The Daily Telegraph
F
E
U
D
A
L
More clues leading to the result FEUDAL
- Row with almost everyone about old-fashioned system of society
- Like a social structure expected to return in autumn? Not entirely
- Part of Venezuela due for uprising, like most of Europe once
- Expected to come back into season, almost like medieval society
- Old-fashioned doctor leaves dreadful drunk
- System taking of vassal a due fraction in return
- Old-fashioned fellow orchestrating a duel
- Gangster sustains bitter disagreement concerning tenure
- Fight, to a man, being unenlightened
- Old-fashioned male taking heart after conflict
Buddy explains!

The straight part
We know that "System" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case system = feudal.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 FEUDAL mean?
FEUDAL adjective- Of, or relating to feudalism.