The most common and most likely answer to this crossword clue is the 7 letter word DOLTISH
I've seen this crossword clue in The Guardian
D
O
L
T
I
S
H
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 = doltish.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 DOLTISH mean?
DOLTISH adjective- Like a dolt; dull in intellect; stupid.