The most common and most likely answer to this crossword clue is the 4 letter word ORLE
O
R
L
E
Buddy explains!
The straight part
We know that "Border" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case border = orle.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 ORLE mean?
ORLE noun- A bordure that runs around the outline of a shield without touching the edge
- The wreath, or chaplet, surmounting or encircling the helmet of a knight and bearing the crest; a torse
- A fillet under the ovolo of a capital