The most common and most likely answer to this crossword clue is the 3 letter word DOH
I've seen this crossword clue in Independent
D
O
H
More clues leading to the result DOH
- Party hard? What an idiot I am!
- Cheat on husband? What an idiot I am!
- Note acknowledgment of stupidity
- Note exclamation of annoyance
- I’ve been stupid to cheat on husband
- I’m an idiot and party hard
- Note on hospital discharge
- Money Sweden brought out, a note
- Stretcher bearer going the wrong way? What an idiot!
- Party hard? I’ve been foolish!
Buddy explains!

The straight part
We know that "I’ve been foolish!" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case i’ve been foolish! = doh.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 DOH mean?
DOH interjection- Expresses frustration or anger, especially at one’s own stupidity.
- An anglicised spelling of do. A syllable used in solfège to represent the first and eighth tonic of a major scale.