The most common and most likely answer to this crossword clue is the 4 letter word GUFF
I've seen this crossword clue in The Daily Telegraph
G
U
F
F
More clues leading to the result GUFF
Buddy explains!
The straight part
We know that "Nonsense" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case nonsense = guff.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 GUFF mean?
GUFF noun- Nonsensical talk or thinking.
- Superfluous information.
- Insolent or otherwise unacceptable remarks.
- A fart; act of breaking wind.
- To fart.
- To mislead.