One could be cavalryman crossing area's frontier (6)
I believe the answer is:
raider
'one' is the definition.
Although both the answer and definition are singular nouns, I don't understand how one could define the other.
I can't explain the rest of the clue.
Can you help me to learn more?
(Other definitions for raider that I've seen before include "person who's involved in sacking?" , "One attacks" , "One plundering" , "One who makes a sudden swift attack" , "Maker of surprise attack" .)