The most common and most likely answer to this crossword clue is the 10 letter word BREASTWORK
I've seen this crossword clue in The Times
B
R
E
A
S
T
W
O
R
K
More clues leading to the result BREASTWORK
Buddy explains!

The straight part
We know that "Fortification" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case fortification = breastwork.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 BREASTWORK mean?
BREASTWORK noun- A fortification consisting of a breast-high bulwark; a parapet.
- A railing on the quarter-deck and forecastle.
- A parapet.
- Breast augmentation.