The most common and most likely answer to this crossword clue is the 5 letter word UHLAN
I've seen this crossword clue in The Times
U
H
L
A
N
More clues leading to the result UHLAN
Buddy explains!
The straight part
We know that "Horseman once" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case horseman once = uhlan.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 UHLAN mean?
UHLAN noun- A lancer, a soldier armed with a lance in a former light cavalry unit of the Polish, Prussian/German, Austrian, and Russian armies.