The most common and most likely answer to this crossword clue is the 3 letter word HUH
I've seen this crossword clue in Financial Times
H
U
H
Buddy explains!
The straight part
We know that "what" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case what = huh.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 HUH mean?
HUH interjection- (with falling pitch) used to express amusement or subtle surprise.
- Used to express doubt or confusion.
- (with rising pitch) Used to reinforce a question.
- (with falling pitch) Used either to belittle the issuer of a statement/question, or sarcastically to indicate utter agreement, and that the statement being responded to is an extreme understatement. The intonation is changed to distinguish between the two meanings - implied dullness for belittlement, and feigned surprise for utter agreement.
- (with rising pitch) Used to indicate that one did not hear what was said.
- (with falling pitch) Used to create a tag question.