The most common and most likely answer to this crossword clue is the 13 letter word FIDDLE-FADDLE
I've seen this crossword clue in The Guardian
F
I
D
D
L
E
-
F
A
D
D
L
E
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 = fiddle-faddle.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 FIDDLE-FADDLE mean?
FIDDLE-FADDLE noun- Nonsense
- To trifle; to dally.