The most common and most likely answer to this crossword clue is the 4 letter word MUSS
M
U
S
S
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The straight part
We know that "conflict once" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case conflict once = muss.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 MUSS mean?
MUSS noun- A mess (disagreeable mixture or confusion of things; disorder)
- A scramble, as when small objects are thrown down, to be taken by those who can seize them; a confused struggle.
- To rumple, tousle or make (something) untidy.
- A term of endearment.