The most common and most likely answer to this crossword clue is the 8 letter word OUTLAWED
I've seen this crossword clue in Independent
O
U
T
L
A
W
E
D
Buddy explains!

The straight part
We know that "banned" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case banned = outlawed.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 OUTLAWED mean?
OUTLAWED verb- To declare illegal.
- To place a ban upon.
- To remove from legal jurisdiction or enforcement.
- To deprive of legal force.
- (of an act) put beyond the limits of the law, forbidden, against the law.
- (of a person) put beyond the limits of the law, declared an outlaw or declared a criminal.