The most common and most likely answer to this crossword clue is the 3 letter word NOT
I've seen this crossword clue in The Times
N
O
T
Buddy explains!

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 NOT mean?
NOT noun- A unary operation on logical values that changes true to false, and false to true.
- Negates the meaning of the modified verb.
- To no degree.
- (understatement, litotes) Used to modify superlatives to indicate the opposite or near opposite, often in a form of understatement.
- And not.
- (1990s) Used to indicate that the previous phrase was meant sarcastically or ironically.