The most common and most likely answer to this crossword clue is the 8 letter word MINDLESS
I've seen this crossword clue in The Sunday Telegraph
M
I
N
D
L
E
S
S
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 = mindless.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 MINDLESS mean?
MINDLESS adjective- Showing a lack of forethought or sense.
- Having no sensible meaning or purpose.
- Heedless.
- (of a thing done) Overly repetitive and unchallenging.
- Lacking a mind.