The most common and most likely answer to this crossword clue is the 10 letter word EMBANKMENT
I've seen this crossword clue in Financial Times
E
M
B
A
N
K
M
E
N
T
Buddy explains!

The straight part
We know that "rampart" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case rampart = embankment.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 EMBANKMENT mean?
EMBANKMENT noun- A long mound of earth, stone, or similar material, usually built for purposes such as to hold back or store water, for protection from weather or enemies, or to support a road or railway.