The most common and most likely answer to this crossword clue is the 5 plus 1 letter word OOPS} –
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Buddy explains!

The straight part
We know that "how careless!" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case how careless! = oops} –.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 OOPS} – mean?
OOPS} – noun- A minor mistake or unforseen difficulty.
- To make a mistake; to blunder.
- Acknowledging a mistake.