The most common and most likely answer to this crossword clue is the 14 letter word HADRIAN’S WALL
I've seen this crossword clue in The Guardian
H
A
D
R
I
A
N
â
€
™
S
Â
W
A
L
L
Buddy explains!
The straight part
We know that "protection across the country" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case protection across the country = hadrian’s wall.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.