The most common and most likely answer to this crossword clue is the 6 letter word IDIOCY
I've seen this crossword clue in The Sunday Telegraph
I
D
I
O
C
Y
More clues leading to the result IDIOCY
- Stupidity of ten caught in awful DIY
- I would love to interrupt hostile folly
- Frozen walls start to damage one old folly
- I do shiver in very cold folly
- One officer occupying self- assembly folly
- The presidio cynically admits stupidity
- I regularly overlook porch during home improvements, showing stupidity
- Old Conservative engaged in independent activity at home? That’s folly
- Independent princess coy about stupidity
- I had nothing invested in very unfriendly and stupid behaviour
Buddy explains!

The straight part
We know that "stupid behaviour" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case stupid behaviour = idiocy.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 IDIOCY mean?
IDIOCY noun- The state or condition of being an idiot; the quality of having an intelligence level far below average; mental retardation.
- An act lacking intelligence or sense; an instance of senselessness; extremely foolish behaviour.