The most common and most likely answer to this crossword clue is the 13 letter word FOOLHARDINESS
F
O
O
L
H
A
R
D
I
N
E
S
S
More clues leading to the result FOOLHARDINESS
Buddy explains!
The straight part
We know that "Rash behaviour" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case rash behaviour = foolhardiness.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.