The most common and most likely answer to this crossword clue is the 6 letter word GALOPS
I've seen this crossword clue in Financial Times
G
A
L
O
P
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More clues leading to the result GALOPS
Buddy explains!

The straight part
We know that "Dances once" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case dances once = galops.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 GALOPS mean?
GALOPS noun- A lively French country dance of the nineteenth century, a forerunner of the polka, combining a glissade with a chassé on alternate feet, usually in a fast 2/4 time.
- The music for a dance of this kind.