The most common and most likely answer to this crossword clue is the 13 letter word HIEROGLYPHICS
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More clues leading to the result HIEROGLYPHICS
Buddy explains!

The straight part
We know that "Foreign characters" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case foreign characters = hieroglyphics.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 HIEROGLYPHICS mean?
HIEROGLYPHICS noun- (chiefly in the plural) a writing system of ancient Egypt, Minoans, Maya and other civilizations, using pictorial symbols to represent individual sounds as a rebus
- Any symbol used in this system; a hieroglyph
- (by extension) undecipherable handwriting or secret symbol