The most common and most likely answer to this crossword clue is the 8 letter word ANSERINE
I've seen this crossword clue in The Daily Telegraph
A
N
S
E
R
I
N
E
Buddy explains!
The straight part
We know that "Stupid" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case stupid = anserine.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 ANSERINE mean?
ANSERINE adjective- Silly, foolish, stupid.
- Pertaining to a goose or geese; gooselike.
- A dipeptide compound present in birds and other vertebrates, originally isolated from goose muscle.