The most common and most likely answer to this crossword clue is the 5 letter word UNDID
I've seen this crossword clue in The Times
U
N
D
I
D
More clues leading to the result UNDID
Buddy explains!

The straight part
We know that "Unbuttoned" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case unbuttoned = undid.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 UNDID mean?
UNDID verb- To reverse the effects of an action.
- To unfasten.
- To impoverish or ruin, as in reputation; to cause the downfall of.