Cryptic crossword clue
Post Office New York state’s used for letter delivery system once 4,7

The most common and most likely answer to this crossword clue is the 4 plus 7 letter word PONY EXPRESS

P
O
N
Y
E
X
P
R
E
S
S

Buddy explains!

The straight part
We know that "etter delivery system once" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case etter delivery system once = pony express.
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.

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