The most common and most likely answer to this crossword clue is the 12 letter word CRENELLATION
I've seen this crossword clue in The Daily Telegraph
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More clues leading to the result CRENELLATION
Buddy explains!
The straight part
We know that "battlement" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case battlement = crenellation.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.
Crossword Answer definitions
CRENELLATION noun- A pattern along the top of a parapet (fortified wall), most often in the form of multiple, regular, rectangular spaces in the top of the wall, through which arrows or other weaponry may be shot, especially as used in medieval European architecture.
- The act of crenellating; adding a top row that looks like the top of a medieval castle.