The most common and most likely answer to this crossword clue is the 4 letter word LEET
L
E
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More clues leading to the result LEET
Buddy explains!

The straight part
We know that "Unconventional writing system" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case unconventional writing system = leet.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 LEET mean?
LEET noun- A portion or list, especially a list of candidates for an office; also the candidates themselves.
- To allow to, not to prevent (+ infinitive, but usually without to).
- To leave.
- To allow the release of (a fluid).
- To allow possession of (a property etc.) in exchange for rent.
- To give, grant, or assign, as a work, privilege, or contract; often with out.
- Used to introduce an imperative in the first or third person.
- To cause (+ bare infinitive).
- A regular court, more specifically a court-leet, in which certain lords had jurisdiction over local disputes, or the physical area of this jurisdiction.
- The European pollock.
- An artificial watercourse, canal or aqueduct, but especially a millrace
- A place where roads meet or cross; intersection
- Abbreviation of leetspeak.
- Of or relating to leetspeak.
- Possessing outstanding skill in a field; expert, masterful.
- Having superior social rank over others; upper class, elite.
- Awesome, typically to describe a feat of skill; cool, sweet.