The most common and most likely answer to this crossword clue is the 5 letter word BONER
B
O
N
E
R
Buddy explains!

The straight part
We know that "blunder" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case blunder = boner.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 BONER mean?
BONER noun- An erect penis.
- One who or that which bones (removes bones).
- A blunder; a silly mistake.