The most common and most likely answer to this crossword clue is the 8 letter word BLOCKADE
I've seen this crossword clue in The Guardian
B
L
O
C
K
A
D
E
More clues leading to the result BLOCKADE
- We hear group of countries support barrier
- Report of alliance funding siege
- Cut off as required, keeping curled hair
- Ordered to bring in secure clampdown
- Jam sandwiches ordered? The opposite, being cut off
- Siege ordered to maintain grip
- Ordered to get round secure barrier
- Prevent use of jam with permanent mark on tablecloth there for good
- Cutting edge round old empty cask causes closure
- Ordered to introduce security, to prevent access
Buddy explains!

The straight part
We know that "barrier" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case barrier = blockade.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 BLOCKADE mean?
BLOCKADE noun- The physical blocking or surrounding of a place, especially a port, in order to prevent commerce and traffic in or out.
- (by extension) Any form of formal isolation of something, especially with the force of law or arms.
- The ships or other forces used to effect a naval blockade.
- Preventing an opponent's pawn moving by placing a piece in front of it
- To create a blockade against.