The most common and most likely answer to this crossword clue is the 3 letter word WYN
W
Y
N
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The straight part
We know that "Old letter" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case old letter = wyn.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 WYN mean?
WYN noun- A letter of the Old English alphabet, borrowed from the futhark and used to represent the sound of w; replaced in Middle English times by the digraph uu, which later developed into the letter w.
- A kind of timber truck, or carriage.