The most common and most likely answer to this crossword clue is the 4 plus 2 plus 1 plus 4 plus 2 plus 3 plus 4 letter word ONCE UP A TIME IN THE WEST
I've seen this crossword clue in The Guardian
O
N
C
E
U
P
A
T
I
M
E
I
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H
E
W
E
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T
Buddy explains!
The straight part
We know that "one of 5 down’s" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case one of 5 down’s = once up a time in the west.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.