The most common and most likely answer to this crossword clue is the 7 letter word MISTAKE
I've seen this crossword clue in The Times
M
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More clues leading to the result MISTAKE
Buddy explains!

The straight part
We know that "a blunder" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case a blunder = mistake.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 MISTAKE mean?
MISTAKE noun- An error; a blunder.
- A pitch which was intended to be pitched in a hard-to-hit location, but instead ends up in an easy-to-hit place.
- To understand wrongly, taking one thing or person for another.
- To misunderstand (someone).
- To commit an unintentional error; to do or think something wrong.
- To take or choose wrongly.