The most common and most likely answer to this crossword clue is the 13 letter word UNINTELLIGENT
I've seen this crossword clue in The Sunday Telegraph
U
N
I
N
T
E
L
L
I
G
E
N
T
More clues leading to the result UNINTELLIGENT
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 = unintelligent.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 UNINTELLIGENT mean?
UNINTELLIGENT adjective- Not intelligent.