The most common and most likely answer to this crossword clue is the 4 plus 1 plus 7 letter word DROP A CLANGER
I've seen this crossword clue in Financial Times
D
R
O
P
A
C
L
A
N
G
E
R
More clues leading to the result DROP A CLANGER
Buddy explains!
The straight part
We know that "Do a stupid thing" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case do a stupid thing = drop a clanger.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.