The most common and most likely answer to this crossword clue is the 4 letter word UNTO
U
N
T
O
Buddy explains!

The straight part
We know that "archaic preposition" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case archaic preposition = unto.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 UNTO mean?
UNTO preposition- Up to, indicating a motion towards a thing and then stopping at it.
- To, indicating an indirect object.
- Up to the time or degree that; until.