The most common and most likely answer to this crossword clue is the 12 letter word BLATHERSKITE
I've seen this crossword clue in The Guardian
B
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More clues leading to the result BLATHERSKITE
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 = blatherskite.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 BLATHERSKITE mean?
BLATHERSKITE noun- A voluble purveyor of nonsense; a blusterer.
- A worthless fellow; a deadbeat.
- Nonsense or blather; empty talk.